Derivative Financial Instruments and Hedge Accounting - Summary of Instruments to Hedge Exposures to Changes in Interest Rates and Foreign Currency Risk Associated with Borrowings (Detail) - Financial assets - derivative financial instruments [member] - GBP (£) £ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2025 |
Dec. 31, 2024 |
|
| Derivative financial instruments for interest rate risk [member] | ||
| Disclosure of detailed information about hedging instruments [line items] | ||
| Carrying amount of hedging instruments | £ 0 | £ (1) |
| Change in fair value of hedging instrument used to determine hedge ineffectiveness | 1 | 5 |
| Nominal amounts of hedging instruments | 0 | 166 |
| Derivative financial instruments for currency risk [member] | ||
| Disclosure of detailed information about hedging instruments [line items] | ||
| Carrying amount of hedging instruments | 0 | 21 |
| Change in fair value of hedging instrument used to determine hedge ineffectiveness | (21) | (8) |
| Nominal amounts of hedging instruments | £ 0 | £ 166 |