v3.26.1
Financial risk management - Disclosure of financial instruments by type of interest rate (Details) - Interest rate risk - GBP (£)
£ in Millions
Mar. 31, 2026
Mar. 31, 2025
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic £ (44,160) £ (41,371)
Fixed rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (36,333) (35,875)
Floating rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (3,342) (835)
Inflation linked    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (4,519) (4,695)
Other    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 34 34
Cash and cash equivalents    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 375 1,178
Cash and cash equivalents | Fixed rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 85 131
Cash and cash equivalents | Floating rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 290 1,047
Cash and cash equivalents | Inflation linked    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Cash and cash equivalents | Other    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Financial investments    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 2,453 5,753
Financial investments | Fixed rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Financial investments | Floating rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 2,419 5,719
Financial investments | Inflation linked    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Financial investments | Other    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 34 34
Borrowings    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (46,755) (47,539)
Borrowings | Fixed rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (41,460) (39,847)
Borrowings | Floating rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (836) (3,061)
Borrowings | Inflation linked    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (4,459) (4,631)
Borrowings | Other    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Pre-derivative position    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (43,927) (40,608)
Pre-derivative position | Fixed rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (41,375) (39,716)
Pre-derivative position | Floating rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 1,873 3,705
Pre-derivative position | Inflation linked    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (4,459) (4,631)
Pre-derivative position | Other    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 34 34
Derivative effect    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (233) (763)
Derivative effect | Fixed rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic 5,042 3,841
Derivative effect | Floating rate    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (5,215) (4,540)
Derivative effect | Inflation linked    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic (60) (64)
Derivative effect | Other    
Disclosure of financial instruments by type of interest rate [line items]    
Risk exposure associated with instruments sharing characteristic £ 0 £ 0