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Retirement benefit obligations - Schedule of Principal Assumptions Used in Valuations (Details)
Jun. 30, 2025
Dec. 31, 2024
Schedule of Principal Actuarial and Financial Assumptions Used in Valuations of Defined Benefit Pension Schemes [Line Items]    
Discount rate 5.61% 5.55%
Rate of inflation:    
Rate of salary increases 0.00% 0.00%
Weighted-average rate of increase for pensions in payment 2.67% 2.69%
Retail Price Index (RPI)    
Rate of inflation:    
Prices index 2.75% 2.97%
Consumer Price Index (CPI)    
Rate of inflation:    
Prices index 2.25% 2.52%