Fair values of financial assets and liabilities - Schedule of Movement in Level 3 Financial Liabilities (Details) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
FINANCIAL INSTRUMENTS (Details) - Schedule of Movement in Level 3 Financial Liabilities [Line Items]
At 1 January	£ 810,281
At 31 December	814,621	£ 810,281
Level 3 £m \| Excluding derivatives
FINANCIAL INSTRUMENTS (Details) - Schedule of Movement in Level 3 Financial Liabilities [Line Items]
At 1 January	45	37
Gains recognised in the income statement within other income	(1)	(4)
Additions	0	33
Redemptions	(1)	(3)
Transfers out of the level 3 portfolio	(1)	(18)
At 31 December	42	45
Gains recognised in the income statement, within other income, relating to the change in fair value of those liabilities held at 31 December	£ (1)	£ (4)

- Definition

The amount of liabilities that are: (a) a contractual obligation: (i) to deliver cash or another financial asset to another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially unfavourable to the entity; or (b) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to deliver a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose, rights, options or warrants to acquire a fixed number of the entity’s own equity instruments for a fixed amount of any currency are equity instruments if the entity offers the rights, options or warrants pro rata to all of its existing owners of the same class of its own non-derivative equity instruments. Also, for those purposes the entity’s own equity instruments do not include puttable financial instruments that are classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. As an exception, an instrument that meets the definition of a financial liability is classified as an equity instrument if it has all the features and meets the conditions in paragraphs 16A-16B or paragraphs 16C-16D of IAS 32. [Refer: Financial instruments, class [member]; Financial assets; Derivatives [member]]

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