v3.26.1
FINANCIAL INSTRUMENTS - Disclosure of Analysis of Fair Value of Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - GBP (£)
£ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2026
Dec. 31, 2025
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 7,703 £ 6,150
Liabilities at fair value 2,754 1,937
Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 1,012 687
Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 1,742 1,250
Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 2,089 870
Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 53 64
Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 5,561 5,216
Level 1    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 5,317 5,017
Liabilities at fair value 0 0
Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Level 1 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Level 1 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Level 1 | Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 5,317 5,017
Level 2    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 2,305 1,039
Liabilities at fair value 2,742 1,924
Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 1,000 674
Level 2 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 1,742 1,250
Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 2,061 840
Level 2 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Level 2 | Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 244 199
Level 3    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 81 94
Liabilities at fair value 12 13
Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 12 13
Level 3 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 28 30
Level 3 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 53 64
Level 3 | Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 0 £ 0
Exchange rate contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Valuation technique A A
Liabilities at fair value £ 379 £ 512
Exchange rate contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 501 £ 671
Valuation technique A A
Exchange rate contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value £ 0 £ 0
Exchange rate contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Exchange rate contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 379 512
Exchange rate contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 501 671
Exchange rate contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Exchange rate contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 0 £ 0
Interest rate contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Valuation technique A & C A & C
Liabilities at fair value £ 1,450 £ 1,182
Interest rate contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 2,340 £ 1,078
Valuation technique A & C A & C
Interest rate contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value £ 0 £ 0
Interest rate contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Interest rate contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 1,450 1,182
Interest rate contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 2,340 1,078
Interest rate contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Interest rate contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 0 £ 0
Inflation rate contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Valuation technique A A
Liabilities at fair value £ 19 £ 30
Inflation rate contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 18 £ 85
Valuation technique A A
Inflation rate contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value £ 0 £ 0
Inflation rate contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Inflation rate contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 19 30
Inflation rate contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 18 85
Inflation rate contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Inflation rate contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 0 £ 0
Equity and credit contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Valuation technique B & D B & D
Liabilities at fair value £ 21 £ 21
Equity and credit contracts | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 87 £ 94
Valuation technique B & D B & D
Equity and credit contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value £ 0 £ 0
Equity and credit contracts | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Equity and credit contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 9 8
Equity and credit contracts | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 59 64
Equity and credit contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 12 13
Equity and credit contracts | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 28 30
Netting | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value (857) (1,058)
Netting | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value (857) (1,058)
Netting | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Netting | Level 1 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Netting | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value (857) (1,058)
Netting | Level 2 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value (857) (1,058)
Netting | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Netting | Level 3 | Derivative financial instruments    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Loans and advances to customers: | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 31 £ 40
Valuation technique A A
Loans and advances to customers: | Level 1 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 0 £ 0
Loans and advances to customers: | Level 2 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Loans and advances to customers: | Level 3 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 31 40
Debt securities and other debt instruments | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 22 £ 24
Valuation technique A, B & D A, B & D
Debt securities and other debt instruments | Level 1 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 0 £ 0
Debt securities and other debt instruments | Level 2 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 0 0
Debt securities and other debt instruments | Level 3 | Other financial assets at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 22 24
Debt securities | Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 5,561 £ 5,216
Valuation technique D D
Debt securities | Level 1 | Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 5,317 £ 5,017
Debt securities | Level 2 | Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value 244 199
Debt securities | Level 3 | Financial assets at FVOCI    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Assets at fair value £ 0 £ 0
Debt securities in issue | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Valuation technique A A
Liabilities at fair value £ 334 £ 331
Debt securities in issue | Level 1 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Debt securities in issue | Level 2 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 334 331
Debt securities in issue | Level 3 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value £ 0 £ 0
Structured deposits | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Valuation technique A A
Liabilities at fair value £ 1,323 £ 824
Structured deposits | Level 1 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Structured deposits | Level 2 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 1,323 824
Structured deposits | Level 3 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value £ 0 £ 0
Zero Amortising Guaranteed Notes | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Valuation technique D D
Liabilities at fair value £ 85 £ 95
Zero Amortising Guaranteed Notes | Level 1 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 0 0
Zero Amortising Guaranteed Notes | Level 2 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value 85 95
Zero Amortising Guaranteed Notes | Level 3 | Other financial liabilities at FVTPL    
Disclosure Of Fair Value Measurements Of Assets And Liabilities [Line Items]    
Liabilities at fair value £ 0 £ 0