Subsidiary undertakings, joint ventures and associates - Disclosure of interests in joint ventures (Details)	12 Months Ended
	Mar. 31, 2026 £ / shares shares	Mar. 31, 2026 € / shares shares
Disclosure of joint ventures [line items]
Par value per share (EUR/GBP per share) \| £ / shares	£ 0.1243129
BritNed Development Limited
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture	50.00%
BritNed Development Limited \| National Grid Interconnector Holdings Limited \| C Ordinary shares
Disclosure of joint ventures [line items]
Number of shares held in joint venture (in shares)	284,500,000	284,500,000
Par value per share (EUR/GBP per share) \| € / shares		€ 0.2
BritNed Development Limited \| National Grid Interconnector Holdings Limited \| A Ordinary shares
Disclosure of joint ventures [line items]
Number of shares held in joint venture (in shares)	1	1
Par value per share (EUR/GBP per share) \| £ / shares	£ 1.00
National Places LLP
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture	50.00%
Nemo Link Limited
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture	50.00%
Nemo Link Limited \| National Grid Interconnector Holdings Limited
Disclosure of joint ventures [line items]
Number of shares held in joint venture (in shares)	121,140,801	121,140,801
Par value per share (EUR/GBP per share) \| € / shares		€ 1
Community Offshore Wind LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture	27.27%
LI Energy Storage System, LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture	50.00%
LI Solar Generation, LLC
Disclosure of joint ventures [line items]
Proportion of ownership interest in joint venture	50.00%