v3.24.1.1.u2
Exceptional items and remeasurements - Narrative (Details) - GBP (£)
£ in Millions
 1 Months Ended 12 Months Ended 17 Months Ended 24 Months Ended
Jan. 31, 2023
Oct. 07, 2022
May 25, 2022
Mar. 31, 2022
Mar. 15, 2022
Nov. 30, 2021
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2022
Mar. 31, 2023
Mar. 31, 2023
Disclosure Of Exceptional Items [Line Items]                      
Cash outflow for transaction and separation costs             £ 33        
Cost efficiency programme, annual savings, target           £ 400          
Cost efficiency programme, period           3 years          
Cash outflows relating to restructuring activities, operating activities             53 £ 85      
Cash inflows relating to insurance recovery             92 79      
Net assets             29,892 29,562   £ 29,562 £ 29,562
Revenue             19,850 21,659 £ 18,449    
Other provisions       £ 2,539     3,109 2,642 2,539 2,642 2,642
St William Homes LLP                      
Disclosure Of Exceptional Items [Line Items]                      
Proportion of ownership interest disposed         50.00%            
Cash proceeds         £ 413            
Millennium Pipeline Company, LLC                      
Disclosure Of Exceptional Items [Line Items]                      
Consideration received   £ 497                  
Proportion of ownership interest disposed   26.25%                  
US                      
Disclosure Of Exceptional Items [Line Items]                      
Revenue             10,787 £ 12,048 £ 10,646    
Other environment related provision                      
Disclosure Of Exceptional Items [Line Items]                      
Real discount rate (in percent)               1.50% 0.50%    
Weighted average duration of cash flows               10 years 6 months      
Other provisions       1,877     £ 2,418 £ 1,891 £ 1,877 1,891 1,891
Other environment related provision | US                      
Disclosure Of Exceptional Items [Line Items]                      
Real discount rate (in percent)             1.50% 1.50%      
Weighted average duration of cash flows             10 years        
Other provisions             £ 2,310 £ 1,768   1,768 1,768
Other environment related provision | UK                      
Disclosure Of Exceptional Items [Line Items]                      
Real discount rate (in percent)             1.00% 1.50%      
Other provisions             £ 108 £ 123   123 123
The Narragansett Electric Company                      
Disclosure Of Exceptional Items [Line Items]                      
Consideration received     £ 3,100                
Net assets     £ 2,700                
UK Gas Transmission business                      
Disclosure Of Exceptional Items [Line Items]                      
Proportion of ownership interest disposed 100.00%             60.00%      
Cash proceeds £ 4,000                    
Exceptional items                      
Disclosure Of Exceptional Items [Line Items]                      
Transaction, separation and integration costs             44 £ 117 223   £ 340
Cash outflow for transaction and separation costs               84 196    
Cost efficiency programme             65 100 42 £ 142  
Income from reimbursements under insurance policies             92 130 0    
Changes in environmental provisions             (496) 176 0    
New operating model implementation costs             0 0 24    
Cash outflow for new operating model implementation costs and cost efficiency programme                 48    
Environmental insurance recovery             0 0 38    
Exceptional items | St William Homes LLP                      
Disclosure Of Exceptional Items [Line Items]                      
Net gain on disposal             0 0 228    
Revenue             0 0 189    
Exceptional items | Millennium Pipeline Company, LLC                      
Disclosure Of Exceptional Items [Line Items]                      
Net gain on disposal             0 335 0    
Proportion of ownership interest disposed   26.25%                  
Exceptional items | UK                      
Disclosure Of Exceptional Items [Line Items]                      
Deferred tax expense (income) relating to tax rate changes or imposition of new taxes       £ 458              
Exceptional items | Other environment related provision | US                      
Disclosure Of Exceptional Items [Line Items]                      
Changes in environmental provisions               165      
Exceptional items | Other environment related provision | UK                      
Disclosure Of Exceptional Items [Line Items]                      
Changes in environmental provisions               11      
Exceptional items | The Narragansett Electric Company                      
Disclosure Of Exceptional Items [Line Items]                      
Transaction, separation and integration costs             11 39      
Net gain on disposal             0 511 £ 0    
Exceptional items | UK Gas Transmission business                      
Disclosure Of Exceptional Items [Line Items]                      
Transaction, separation and integration costs             6 38      
Exceptional items | National Grid Electricity Distribution                      
Disclosure Of Exceptional Items [Line Items]                      
Transaction, separation and integration costs             £ 27 £ 40