v3.25.4
Cash and Cash Equivalents (Excluding Overdrafts) - Additional Information (Detail) - GBP (£)
£ in Millions
Dec. 31, 2025
Dec. 31, 2024
Cash and cash equivalents [line items]    
Cash and cash equivalents £ 333 £ 543
Financial liabilities 0 2
Level 1 of fair value hierarchy [member] | Money market funds [member]    
Cash and cash equivalents [line items]    
Cash and cash equivalents £ 11 £ 62
US Dollars [member]    
Cash and cash equivalents [line items]    
Cash and cash equivalents currency split 29.00% 33.00%
Sterling [member]    
Cash and cash equivalents [line items]    
Cash and cash equivalents currency split 14.00% 27.00%
Other currency [member]    
Cash and cash equivalents [line items]    
Cash and cash equivalents currency split 57.00% 40.00%