Property, plant and equipment - Disclosure of information in relation to right-of-use assets (Details) - GBP (£) £ in Millions	12 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Property, plant and equipment [abstract]
Interest expense on lease liabilities	£ (41)	£ (40)
Lease income	385	406
Expense relating to short-term and low-value leases	(27)	(24)
Variable lease payments included in lease income	£ 364	£ 384

The amount of operating lease income. Operating lease is a lease that does not transfer substantially all the risks and rewards incidental to ownership of an underlying asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2025-01-01
-Paragraph 90
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2025-en-r&anchor=para_90_b&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2027-01-01
-Name IFRS
-Number 19
-IssueDate 2025-01-01
-Paragraph 125
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=19&code=ifrs-tx-2025-en-r&anchor=para_125&doctype=Standard
-URIDate 2025-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2027-01-01
-Name IFRS
-Number 19
-IssueDate 2025-01-01
-Paragraph 125
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=19&code=ifrs-tx-2025-en-r&anchor=para_125&doctype=Standard
-URIDate 2025-03-27

Name:	ifrs-full_OperatingLeaseIncome
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration