v3.26.1
Financial risk management - Disclosure of nature and extent of risks arising from financial instruments (Details) - Currency risk - GBP (£)
£ in Millions
Mar. 31, 2026
Mar. 31, 2025
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic £ (44,160) £ (41,371)
Cash and cash equivalents    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 375 1,178
Financial investments    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 2,453 5,753
Borrowings    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (46,755) (47,539)
Pre-derivative position    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (43,927) (40,608)
Derivative effect    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (233) (763)
Trade and other receivables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 2,671 2,696
Other non-current assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 291 299
Trade and other payables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (4,501) (3,908)
Other non-current liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (707) (467)
GBP    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (18,089) (16,276)
GBP | Cash and cash equivalents    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 287 1,047
GBP | Financial investments    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 1,769 5,129
GBP | Borrowings    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (12,161) (13,913)
GBP | Pre-derivative position    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (10,105) (7,737)
GBP | Derivative effect    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (7,984) (8,539)
GBP | Trade and other receivables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 353 424
GBP | Other non-current assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 223 243
GBP | Trade and other payables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (1,868) (1,359)
GBP | Other non-current liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (360) (171)
Euro    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 1,472 918
Euro | Cash and cash equivalents    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 2 0
Euro | Financial investments    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Euro | Borrowings    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (13,552) (12,968)
Euro | Pre-derivative position    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (13,550) (12,968)
Euro | Derivative effect    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 15,022 13,886
Euro | Trade and other receivables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Euro | Other non-current assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Euro | Trade and other payables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Euro | Other non-current liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
USD    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (27,840) (26,217)
USD | Cash and cash equivalents    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 86 131
USD | Financial investments    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 684 624
USD | Borrowings    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (19,469) (19,217)
USD | Pre-derivative position    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (18,699) (18,462)
USD | Derivative effect    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (9,141) (7,755)
USD | Trade and other receivables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 2,318 2,272
USD | Other non-current assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 68 56
USD | Trade and other payables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (2,633) (2,549)
USD | Other non-current liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (347) (296)
Other    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 297 204
Other | Cash and cash equivalents    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Other | Financial investments    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Other | Borrowings    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (1,573) (1,441)
Other | Pre-derivative position    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (1,573) (1,441)
Other | Derivative effect    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 1,870 1,645
Other | Trade and other receivables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Other | Other non-current assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Other | Trade and other payables    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Other | Other non-current liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic £ 0 £ 0