Derivative Financial Instruments and Hedge Accounting - Summary of Amounts at the Reporting Rate Relating to Items Designated as Hedge Items (Detail) - Financial liabilities - borrowings [member] - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Disclosure of detailed information about hedged items [line items]
Hedge ineffectiveness	£ 0	£ 0
Interest rate risk [member]
Disclosure of detailed information about hedged items [line items]
Carrying amount of hedged items	0	(166)
Accumulated amount of fair value hedge adjustments on the hedged item included in the carrying amount	0	1
Change in fair value of hedged item used to determine hedge ineffectiveness	0	(5)
Hedge ineffectiveness	0	0
Currency risk [member]
Disclosure of detailed information about hedged items [line items]
Carrying amount of hedged items	0	(166)
Accumulated amount of fair value hedge adjustments on the hedged item included in the carrying amount	0
Change in fair value of hedged item used to determine hedge ineffectiveness	0	8
Hedge ineffectiveness	£ 0	£ 0

Derivative Financial Instruments and Hedge Accounting - Summary of Amounts at the Reporting Rate Relating to Items Designated as Hedge Items (Detail) - Financial liabilities - borrowings [member] - GBP (£)
£ in Millions

12 Months Ended

Dec. 31, 2025

Dec. 31, 2024

Disclosure of detailed information about hedged items [line items]

Hedge ineffectiveness

£ 0

Interest rate risk [member]

Disclosure of detailed information about hedged items [line items]

Carrying amount of hedged items

(166)

Accumulated amount of fair value hedge adjustments on the hedged item included in the carrying amount

Change in fair value of hedged item used to determine hedge ineffectiveness

(5)

Hedge ineffectiveness

Currency risk [member]

Disclosure of detailed information about hedged items [line items]

Carrying amount of hedged items

(166)

Accumulated amount of fair value hedge adjustments on the hedged item included in the carrying amount

Change in fair value of hedged item used to determine hedge ineffectiveness

Hedge ineffectiveness

£ 0

- Definition

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.

+ References + Details