Fair values of financial assets and liabilities - Schedule of Sensitivity of Level 3 Valuations (Details) - Level 3 £m - Recurring fair value measurement - GBP (£) £ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2026	Dec. 31, 2025
Credit spreads \| Financial liabilities at fair value through profit or loss \| Securitisation notes and other
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, liabilities [abstract]
Max up	0.50%	0.50%
Max down	(0.50%)	(0.50%)
Favourable changes	£ 1	£ 2
Unfavourable changes	£ (1)	£ (2)
HPI growth \| Derivative financial liabilities \| Shared appreciation rights
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, liabilities [abstract]
Max up	1.00%	1.00%
Max down	(1.00%)	(1.00%)
Favourable changes	£ 9	£ 11
Unfavourable changes	£ (8)	£ (10)
Interest rate ATM volatility \| Derivative financial liabilities \| Interest rate derivatives
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, liabilities [abstract]
Max up	0.04%	0.04%
Max down	(0.04%)	(0.04%)
Favourable changes	£ 0	£ 0
Unfavourable changes	£ 0	£ 0
Financial assets at fair value through profit or loss \| Loans and advances to customers \| Credit spreads
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, assets [abstract]
Max up	1.15%	1.15%
Max down	(1.15%)	(1.15%)
Favourable changes	£ 5	£ 5
Unfavourable changes	£ (5)	£ (5)
Financial assets at fair value through profit or loss \| Loans and advances to customers \| HPI growth
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, assets [abstract]
Max up	1.00%	1.00%
Max down	(1.00%)	(1.00%)
Favourable changes	£ 11	£ 14
Unfavourable changes	£ (6)	£ (12)
Financial assets at fair value through profit or loss \| Debt securities \| Price
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, assets [abstract]
Max up	10.00%
Max down	(10.00%)
Favourable changes	£ 0
Unfavourable changes	£ 0
Financial assets at fair value through profit or loss \| Equity shares \| Price
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, assets [abstract]
Max up	46.00%	31.00%
Max down	(46.00%)	(31.00%)
Favourable changes	£ 1	£ 1
Unfavourable changes	£ (1)	£ (1)
Financial assets at fair value through other comprehensive income \| Debt securities \| Credit spreads
Increase (decrease) in fair value measurement for sensitivity analyses in which inputs are changed individually, assets [abstract]
Max up	0.75%	0.75%
Max down	(0.75%)	(0.75%)
Favourable changes	£ 1	£ 2
Unfavourable changes	£ (1)	£ (2)

The amount of increase (decrease) in the fair value measurement of assets due to a reasonably possible decrease in an unobservable input.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
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-IssueDate 2025-01-01
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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