v3.26.1
Financial risk management - Disclosure of effects of hedge accounting on financial position and year-to-date performance for each type of hedge (Details)
£ in Millions
 12 Months Ended
Mar. 31, 2026
GBP (£)
Mar. 31, 2025
GBP (£)
Foreign currency and interest rate risk | Fair value hedges    
Disclosure of detailed information about hedging instruments [line items]    
Hedging instrument notional (8,098) (6,767)
Balance of fair value hedge adjustments in borrowings, Continuing hedges £ 660 £ 756
Balance of fair value hedge adjustments in borrowings, Discontinued hedges (21) (25)
Change in value used for calculating ineffectiveness, Hedged item (104) 106
Change in value used for calculating ineffectiveness, Hedging instrument 97 (94)
Hedge ineffectiveness (7) 12
Foreign currency and interest rate risk | Fair value hedges | Borrowings    
Disclosure of detailed information about hedging instruments [line items]    
Hedged borrowings 7,767 6,414
Foreign currency and interest rate risk | Fair value hedges | Borrowings | Current liabilities    
Disclosure of detailed information about hedging instruments [line items]    
Hedged borrowings 314 118
Foreign currency and interest rate risk | Fair value hedges | Borrowings | Non-current liabilities    
Disclosure of detailed information about hedging instruments [line items]    
Hedged borrowings £ 7,453 £ 6,296
Foreign currency and interest rate risk | Cash flow hedges    
Disclosure of detailed information about hedging instruments [line items]    
Hedging instrument notional (13,762) (14,769)
Balance in cash flow hedge reserve, Continuing hedges £ 258 £ 376
Balance in cash flow hedge reserve, Discontinued hedges 0 0
Change in value used for calculating ineffectiveness, Hedged item (359) (33)
Change in value used for calculating ineffectiveness, Hedging instrument 356 27
Hedge ineffectiveness £ (3) £ (6)
Currency risk | Cash flow hedges    
Disclosure of detailed information about hedging instruments [line items]    
Hedging instrument notional (2,715) (1,907)
Balance in cash flow hedge reserve, Continuing hedges £ (22) £ (43)
Balance in cash flow hedge reserve, Discontinued hedges 0 0
Change in value used for calculating ineffectiveness, Hedged item (21) 12
Change in value used for calculating ineffectiveness, Hedging instrument 21 (12)
Hedge ineffectiveness £ 0 £ 0
Currency risk | Net investment hedges    
Disclosure of detailed information about hedging instruments [line items]    
Hedging instrument notional (3,210) (2,641)
Balance in translation reserve, Continuing hedges £ 76 £ 55
Balance in translation reserve, Discontinued hedges (2,520) (2,523)
Change in value used for calculating ineffectiveness, Hedged item (32) (56)
Change in value used for calculating ineffectiveness, Hedging instrument 32 56
Hedge ineffectiveness £ 0 £ 0