RETIREMENT BENEFIT PLANS - Summary of Actuarial Assumption Sensitivities (Details) - GBP (£) £ in Millions |
6 Months Ended | |
|---|---|---|
Jun. 30, 2026 |
Dec. 31, 2025 |
|
| Discount rate | ||
| Disclosure of sensitivity analysis for actuarial assumptions [line items] | ||
| Percentage of reasonably possible increase in actuarial assumption | 0.50% | |
| Increase (decrease) in defined benefit obligation | £ (360) | £ (377) |
| General price inflation | ||
| Disclosure of sensitivity analysis for actuarial assumptions [line items] | ||
| Percentage of reasonably possible increase in actuarial assumption | 0.50% | |
| Increase (decrease) in defined benefit obligation | £ 290 | 305 |
| Mortality | ||
| Disclosure of sensitivity analysis for actuarial assumptions [line items] | ||
| Description of changes in methods and assumptions | Each additional year of longevity assumed | |
| Increase (decrease) in defined benefit obligation | £ 185 | £ 195 |