Retirement Benefit and Other Post-retirement Obligations - Summary of Effect Percentage of Discount Rate Increase (Decrease) on Defined Benefit Obligation and Total Pension Expense (Detail) - Rate used to discount plan liabilities [member] £ in Millions |
Dec. 31, 2025
GBP (£)
|
|---|---|
| UK Group plan [member] | |
| Disclosure of sensitivity analysis for actuarial assumptions [line items] | |
| (Decrease) /increase in defined benefit obligation, 1% increase in discount rate | £ (151) |
| (Decrease) /increase in defined benefit obligation, 1% decrease in discount rate | 181 |
| US defined benefit pension plans [member] | |
| Disclosure of sensitivity analysis for actuarial assumptions [line items] | |
| (Decrease) /increase in defined benefit obligation, 1% increase in discount rate | (1) |
| (Decrease) /increase in defined benefit obligation, 1% decrease in discount rate | £ 1 |