As I recall, there were two reasons the UK went over to retailing petrol (gasoline) in litres.
1, The nation's ancient stock of (mechanical) petrol pumps were not suited to recording sales where the unit price (one gallon) was greater than £1- (and prices were heading that way!)
2, Once changed, an increase of the prices of 1p per unit (litre) looks much better than an increase of 4.4p per unit (gallon)