Fair values of financial assets and liabilities - Schedule of Financial Liabilities Carried at Fair Value by Valuation Hierarchy (Details) - GBP (£) £ in Millions	Jun. 30, 2026	Dec. 31, 2025
Disclosure of fair value measurement of liabilities [line items]
Financial liabilities at fair value through profit or loss	£ 4,238	£ 4,243
Derivative financial instruments	4,392	4,286
Total financial liabilities carried at fair value	8,630	8,529
Level 1 £m
Disclosure of fair value measurement of liabilities [line items]
Financial liabilities at fair value through profit or loss	0	0
Derivative financial instruments	0	0
Total financial liabilities carried at fair value	0	0
Level 2 £m
Disclosure of fair value measurement of liabilities [line items]
Financial liabilities at fair value through profit or loss	4,221	4,226
Derivative financial instruments	4,285	4,168
Total financial liabilities carried at fair value	8,506	8,394
Level 3 £m
Disclosure of fair value measurement of liabilities [line items]
Financial liabilities at fair value through profit or loss	17	17
Derivative financial instruments	107	118
Total financial liabilities carried at fair value	£ 124	£ 135

- Definition

The amount of financial liabilities that meet either of the following conditions: (a) they meet the definition of held for trading; or (b) upon initial recognition they are designated by the entity as at fair value through profit or loss. An entity may use this designation only when permitted by paragraph 4.3.5 of IFRS 9 (embedded derivatives) or when doing so results in more relevant information, because either: (a) it eliminates or significantly reduces a measurement or recognition inconsistency (sometimes referred to as ‘an accounting mismatch’) that would otherwise arise from measuring assets or liabilities or recognising the gains and losses on them on different bases; or (b) a group of financial liabilities or financial assets and financial liabilities is managed and its performance is evaluated on a fair value basis, in accordance with a documented risk management or investment strategy, and information about the group is provided internally on that basis to the entity’s key management personnel (as defined in IAS 24). [Refer: At fair value [member]; Key management personnel of entity or parent [member]; Derivatives [member]; Financial assets; Financial liabilities]

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