Diesel costs more than petrol/gas in the UK
Petrol/gas £1-21/Litre = $9-64 / gallon
Diesel £1-25/litre = $9-96 / gallon
Most of that is tax
Check your math
1 US Gallon = 3.78 litres
1GBP = $1.47 today
so 1 litre of gasoline in the UK @ GBP1.21 = $6.72 / gallon.