Derivative financial instruments - Net investment hedge (Details) - Net investment hedge € in Millions, £ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2024 GBP (£) $ / £ USD ($) ¥ / £ € / £ EUR (€)	Dec. 31, 2023 GBP (£) $ / £ EUR (€) € / £ USD ($) ¥ / £	Dec. 31, 2022 GBP (£)
Derivative financial instruments
Hedge ratio	1
Ineffectiveness	£ 0	£ 0
Gain (loss) in net investment hedge reserves related to continuing hedges	6	16	£ (91)
Gain (loss) in net investment hedge reserves related to discontinued hedges	(7)	0	£ 0
Cross-currency swaps \| US dollar
Derivative financial instruments
Carrying amount	£ 4	£ 9
Notional amount	(1,300)	(1,641)
Hedge ratio	1	1
Change in fair value of outstanding instrument	£ (5)	£ 114
Change in fair value of hedged item	£ (5)	£ 114
Weighted average foreign exchange rate for the year \| $ / £	1.241	1.25
Cross-currency swaps \| JPY
Derivative financial instruments
Carrying amount		£ 1
Notional amount	(10)	(11)
Hedge ratio	1	1
Change in fair value of outstanding instrument	£ (1)	£ 1
Change in fair value of hedged item	£ (1)	£ 1
Weighted average foreign exchange rate for the year \| ¥ / £	169.747	167.269
Bonds \| Euro
Derivative financial instruments
Carrying amount	£ (261)	£ (298)
Notional amount	(315)	(343)
Hedge ratio	1	1
Change in fair value of outstanding instrument	£ 16	£ 6
Change in fair value of hedged item	£ 16	£ 6
Weighted average foreign exchange rate for the year \| € / £	1.162	1.162
Term loan \| US dollar
Derivative financial instruments
Carrying amount	£ (436)	£ (360)
Notional amount	(546)	(459)
Hedge ratio	1	1
Change in fair value of outstanding instrument	£ 6	£ 9
Change in fair value of hedged item	£ 6	£ 9
Weighted average foreign exchange rate for the year \| $ / £	1.11	1.11

The average exchange rate used by the entity. Exchange rate is the ratio of exchange for two currencies.

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The gain (loss) on the change in fair value of the hedged item used as a basis for recognising hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item.

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The gain (loss) on the change in fair value of the hedging instrument used as a basis for recognising hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item.

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The gain (loss) recognised in profit or loss on hedge ineffectiveness. [Refer: Gain (loss) on hedge ineffectiveness]

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The amount of a hedging instrument, recognised as an asset. [Refer: Hedging instruments [domain]]

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The amount of a hedging instrument, recognised as a liability. [Refer: Hedging instruments [domain]]

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The nominal amount of a hedging instrument. [Refer: Hedging instruments [domain]]

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A component of equity representing the reserve of exchange differences on translation in relation to continuing hedges. [Refer: Reserve of exchange differences on translation]

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A component of equity representing the reserve of exchange differences on translation in relation to hedging relationships for which hedge accounting is no longer applied. [Refer: Reserve of exchange differences on translation]

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The hedge ratio for currencies.

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