Derivative financial instruments - Schedule of fair values of derivatives by category (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)
Current
Disclosure of detailed information about financial instruments [line items]
Derivative assets	215	113
Derivative liabilities	(268)	(381)
Total	(53)	(268)
Non-current
Disclosure of detailed information about financial instruments [line items]
Derivative assets	623	369
Derivative liabilities	(750)	(821)
Total	(127)	(452)
Financing derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	717	375
Derivative liabilities	(950)	(1,138)
Total	(233)	(763)
Commodity contract derivatives
Disclosure of detailed information about financial instruments [line items]
Derivative assets	121	107
Derivative liabilities	(68)	(64)
Total	£ 53	£ 43

The amount of financial assets classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 55
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_55&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_DerivativeFinancialAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of financial liabilities classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

+ References

+ Details

Name:	ifrs-full_DerivativeFinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfFinancialInstrumentsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Derivative Financial Assets (Liabilities)

+ References

No definition available.

+ Details

Name:	nggtf_DerivativeFinancialAssetsLiabilities
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=nggtf_CurrentDerivativeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=nggtf_NonCurrentDerivativeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=nggtf_FinancingDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=nggtf_CommodityDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: