Employee benefit obligations - Movement in Pension Plan Liabilities (Details) £ in Millions	12 Months Ended
Dec. 31, 2024 GBP (£)	Dec. 31, 2023 GBP (£) defined_benefit_plan	Dec. 31, 2022 GBP (£)
Disclosure of net defined benefit (liability) asset
Plan liabilities at beginning of year	£ 122
Plan liabilities at end of year	114	£ 122
Number of defined benefit pension plans winded up \| defined_benefit_plan		2
Full elimination of associated plan assets and plan liabilities		£ 145
Retirement benefit obligations
Disclosure of net defined benefit (liability) asset
Plan liabilities at beginning of year	381	553	£ 689
Service cost	12	12	11
Interest cost	16	21	16
Effect of changes in demographic assumptions	0	1	0
Effect of changes in financial assumptions	(11)	14	(144)
Effect of experience adjustments	4	1	0
Benefits paid	(33)	(38)	(52)
(Gain)/loss due to exchange rate movements	(2)	(17)	40
Settlement payments	(1)	(163)	(9)
Other	(1)	(3)	2
Plan liabilities at end of year	£ 365	£ 381	£ 553

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 141
-Subparagraph c
-Clause ii
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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 141
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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- 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.

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 141
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes that the entity does not separately disclose in the same statement or note. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 141
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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [domain]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 141
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan in respect of settlements. [Refer: Decrease (increase) in net defined benefit liability (asset) resulting from gains (losses) arising from settlements; Net defined benefit liability (asset); Defined benefit plans [domain]]

+ References

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- Definition

Elimination of Plan Assets and Liabilities

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No definition available.

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- Definition

Experience gain/(loss) arising on the plan liabilities.

+ References

No definition available.

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- Definition

Interest cost on plan liabilities

+ References

No definition available.

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- Definition

Number of Defined Benefit Pension Plans Winded Up

+ References

No definition available.

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