Sensitivities - Schedule of sensitivities on financial instruments (Details) - GBP (£) £ in Millions	12 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Fair value change \| Derivative financial instruments
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, percent	10.00%	10.00%
Change in risk variable, impact on income statement	£ (18)	£ (57)
Change in risk variable, impact on net assets	£ (18)	£ (57)
Fair value change \| Commodity contract derivative liabilities
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, percent	10.00%	10.00%
Change in risk variable, impact on income statement	£ 4	£ 3
Change in risk variable, impact on net assets	£ 4	£ 3
Inflation, UK RPI change \| UK
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, percent	1.00%	1.00%
Change in risk variable, impact on income statement	£ 33	£ 35
Change in risk variable, impact on other equity reserves	£ 0	£ 0
Interest rate risk \| UK
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, percent	1.00%	1.00%
Change in risk variable, impact on income statement	£ 14	£ 13
Change in risk variable, impact on other equity reserves	£ 352	£ 376
Interest rate risk \| US
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, percent	1.00%	1.00%
Change in risk variable, impact on income statement	£ 11	£ 18
Change in risk variable, impact on other equity reserves	151	134
Currency risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, impact on net assets	£ 1,887	£ 1,730
Currency risk \| US
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, percent	10.00%	10.00%
Change in risk variable, impact on income statement	£ 64	£ 69
Change in risk variable, impact on other equity reserves	£ 276	£ 225
Commodity price risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Change in risk variable, percent	10.00%	10.00%
Increase in risk variable, impact on earnings	£ 56	£ 62
Increase in risk variable, impact on net assets	56	62
Decrease in risk variable, impact on earnings	(57)	(61)
Decrease in risk variable, impact on net assets	£ (57)	£ (61)

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

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Change In Risk Variable, Impact On Earnings

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysisForTypesOfMarketRiskReasonablyPossibleChangeInRiskVariableImpactOnEarnings
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Change In Risk Variable, Impact On Net Assets

+ References

No definition available.

+ Details

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

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Change In Risk Variable, Impact On Other Equity Reserves

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysisForTypesOfMarketRiskReasonablyPossibleChangeInRiskVariableImpactOnOtherEquityReserves
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Change In Risk Variable, Percent

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysisForTypesOfMarketRiskReasonablyPossibleChangeInRiskVariablePercent
Namespace Prefix:	nggtf_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Decrease In Risk Variable, Impact On Earnings

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysisForTypesOfMarketRiskReasonablyPossibleDecreaseInRiskVariableImpactOnEarnings
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Decrease In Risk Variable, Impact On Net Assets

+ References

No definition available.

+ Details

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

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Increase In Risk Variable, Impact On Earnings

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysisForTypesOfMarketRiskReasonablyPossibleIncreaseInRiskVariableImpactOnEarnings
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Sensitivity Analysis For Types Of Market Risk, Reasonably Possible Increase In Risk Variable, Impact On Net Assets

+ References

No definition available.

+ Details

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

- Details

Name:	nggtf_OtherSensitivitiesAxis=nggtf_FairValueChangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nggtf_BalanceSheetLocation1Axis=nggtf_DerivativeFinancialInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nggtf_BalanceSheetLocation1Axis=nggtf_CommodityContractDerivativeLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_OtherPriceRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_GeographicalAreasAxis=country_GB
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_InterestRateRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_GeographicalAreasAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_CurrencyRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_CommodityPriceRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: