Derivative Financial Instruments and Hedge Accounting - Summary of Amounts Related to Items Designated as Hedging Instruments (Detail) - GBP (£) £ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2025 |
Dec. 31, 2024 |
|
| Financial liabilities - derivative financial instruments [member] | ||
| Disclosure of detailed information about hedges [line items] | ||
| Carrying amount of hedging instruments | £ 1 | £ (47) |
| Change in value of hedging instrument used to determine hedge ineffectiveness | 48 | (29) |
| Nominal amounts of hedging instruments | 993 | 1,225 |
| Hedging gains/(losses) recognised in OCI | 48 | (29) |
| Hedge ineffectiveness recognised in profit or loss | 0 | 0 |
| Financial liabilities - borrowings [member] | ||
| Disclosure of detailed information about hedges [line items] | ||
| Carrying amount of hedging instruments | 0 | 0 |
| Change in value of hedging instrument used to determine hedge ineffectiveness | 0 | 0 |
| Nominal amounts of hedging instruments | 0 | 0 |
| Hedging gains/(losses) recognised in OCI | 0 | 0 |
| Hedge ineffectiveness recognised in profit or loss | £ 0 | £ 0 |