Net debt - Schedule of balance sheet allocation of derivative balances (Details) - GBP (£) £ in Millions |
Sep. 30, 2026 |
Mar. 31, 2026 |
|---|---|---|
| Disclosure of detailed information about financial instruments [line items] | ||
| Derivative financial assets | £ 720 | £ 838 |
| Derivative financial liabilities | (1,385) | (1,018) |
| Financing derivatives | (665) | (180) |
| Financing derivatives | ||
| Disclosure of detailed information about financial instruments [line items] | ||
| Derivative financial assets | 572 | 717 |
| Derivative financial liabilities | (1,272) | (950) |
| Financing derivatives | (700) | (233) |
| Commodity contract derivatives | ||
| Disclosure of detailed information about financial instruments [line items] | ||
| Derivative financial assets | 148 | 121 |
| Derivative financial liabilities | (113) | (68) |
| Financing derivatives | £ 35 | £ 53 |