Net debt - Schedule of balance sheet allocation of derivative balances (Details) - GBP (£) £ in Millions |
Sep. 30, 2025 |
Mar. 31, 2025 |
|---|---|---|
| Disclosure of detailed information about financial instruments [line items] | ||
| Financial investments | £ 976 | £ 482 |
| Derivative financial liabilities | (913) | (1,202) |
| Financing derivatives | 63 | (720) |
| Financing derivatives | ||
| Disclosure of detailed information about financial instruments [line items] | ||
| Financial investments | 851 | 375 |
| Derivative financial liabilities | (845) | (1,138) |
| Financing derivatives | 6 | (763) |
| Commodity contract derivatives | ||
| Disclosure of detailed information about financial instruments [line items] | ||
| Financial investments | 125 | 107 |
| Derivative financial liabilities | (68) | (64) |
| Financing derivatives | £ 57 | £ 43 |