FINANCIAL INSTRUMENTS - Summary of Fair Value Adjustments (Details) - Risk-related: - GBP (£) £ in Millions |
6 Months Ended | 12 Months Ended |
|---|---|---|
Jun. 30, 2026 |
Dec. 31, 2025 |
|
| Disclosure of financial assets [line items] | ||
| Fair value adjustments | £ 12 | £ 15 |
| - Bid-offer and trade specific adjustments | ||
| Disclosure of financial assets [line items] | ||
| Fair value adjustments | 8 | 9 |
| - Uncertainty | ||
| Disclosure of financial assets [line items] | ||
| Fair value adjustments | 3 | 4 |
| - Credit risk adjustment | ||
| Disclosure of financial assets [line items] | ||
| Fair value adjustments | 1 | 1 |
| - Funding fair value adjustment | ||
| Disclosure of financial assets [line items] | ||
| Fair value adjustments | £ 0 | £ 1 |