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Financial Liabilities - Borrowings - Summary of Present Value of Finance Lease Obligations (Detail) - GBP (£)
£ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about borrowings [line items]    
Present value of finance lease Obligations £ 8 £ 9
In less than one year [member]    
Disclosure of detailed information about borrowings [line items]    
Present value of finance lease Obligations 4 5
One to five years [member]    
Disclosure of detailed information about borrowings [line items]    
Present value of finance lease Obligations £ 4 £ 4