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Financial Liabilities - Borrowings - Summary of Present Value of Finance Lease Obligations (Detail) - GBP (£)
£ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about borrowings [line items]    
Present value of finance lease Obligations £ 5 £ 8
In less than one year [member]    
Disclosure of detailed information about borrowings [line items]    
Present value of finance lease Obligations 3 4
One to five years [member]    
Disclosure of detailed information about borrowings [line items]    
Present value of finance lease Obligations £ 2 £ 4