Fair value measurements and valuation processes - summary of assets and liabilities measured at fair value (Details) - Level 2 - GBP (£) £ in Millions |
Jun. 30, 2026 |
Dec. 31, 2025 |
Jun. 30, 2025 |
|---|---|---|---|
| Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] | |||
| Assets at fair value | £ 239 | £ 297 | £ 443 |
| Liabilities at fair value | 237 | 215 | 338 |
| – interest rate swaps | |||
| Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] | |||
| Assets | 18 | 44 | 59 |
| Liabilities | 129 | 92 | 117 |
| – cross-currency swaps | |||
| Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] | |||
| Assets | 93 | 105 | 110 |
| Liabilities | 7 | 5 | 14 |
| – forward foreign currency contracts | |||
| Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items] | |||
| Assets | 128 | 148 | 274 |
| Liabilities | £ 101 | £ 118 | £ 207 |