v3.26.1
Derivative financial instruments - Schedule of fair values of commodity contract derivatives by type (Details) - GBP (£)
£ in Millions
Mar. 31, 2026
Mar. 31, 2025
Disclosure of detailed information about financial instruments [line items]    
Derivative assets £ 838 £ 482
Derivative liabilities (1,018) (1,202)
Total (180) (720)
Commodity contract derivatives    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 121 107
Derivative liabilities (68) (64)
Total 53 43
Forward purchases of gas    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 0 3
Derivative liabilities 0 (7)
Total 0 (4)
Gas options    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 3 0
Derivative liabilities (3) 0
Total 0 0
Electricity capacity    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 7 2
Derivative liabilities (10) (17)
Total (3) (15)
Electricity swaps    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 110 74
Derivative liabilities (44) (38)
Total 66 36
Electricity options    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 0 1
Derivative liabilities 0 (1)
Total 0 0
Gas swaps    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 0 15
Derivative liabilities (8) (1)
Total (8) 14
Gas options    
Disclosure of detailed information about financial instruments [line items]    
Derivative assets 1 12
Derivative liabilities (3) 0
Total £ (2) £ 12