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Sources of finance - Analysis of Fixed and Floating Rate Debt by Currency (Detail)
€ in Millions, £ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2019
GBP (£)
Dec. 31, 2018
GBP (£)
Dec. 31, 2019
USD ($)
Dec. 31, 2019
EUR (€)
Disclosure of detailed information about financial instruments [line items]        
Borrowings £ 4,272.9 £ 6,217.9    
US Dollar bonds [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings | $     $ 1,563  
US Dollar bonds [member] | Fixed interest rate [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings £ 1,178.2 £ 1,154.8    
Fixed rate [1] 4.06% 4.58% 4.06% 4.06%
Period (months) [1] 95 months 181 months    
US Dollar bonds [member] | Floating interest rate [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings   £ 1,029.6    
Floating basis   LIBOR    
Sterling bonds [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings £ 400.0      
Sterling bonds [member] | Fixed interest rate [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings £ 844.1 £ 1,044.1    
Fixed rate [1] 2.73% 3.43% 2.73% 2.73%
Period (months) [1] 188 months 232 months    
Eurobonds [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings | €       € 3,100
Eurobonds [member] | Fixed interest rate [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings £ 1,777.7 £ 2,425.9    
Fixed rate [1] 2.34% 1.99% 2.34% 2.34%
Period (months) [1] 82 months 75 months    
Eurobonds [member] | Floating interest rate [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings £ 423.3 £ 449.2    
Floating basis EURIBOR EURIBOR    
Period (months) [1] 16 months      
Other bonds [member]        
Disclosure of detailed information about financial instruments [line items]        
Borrowings £ 49.6 £ 114.3    
[1] Weighted average. These rates do not include the effect of gains on interest rate swap terminations that are written to income over the life of the original instrument.