Er the average price in the European union is about $2.05 a litre, the UK is 21 cents more expensive but other countries like France, Italy, Netherlands, Sweden, Finland are more expensive then the UK. Norway which produces quite a lot of oil, has the 2nd highest fuel prices in the world at $2.56 a litre. America really has artificially low fuel prices, it being $1.02 a litre, Iraq is slightly cheaper at $1. Only middle eastern and other oil producing nations really have lower prices. Venezuela has petrol at 2 cents a litre... To be fair, you will probably drive shorter distances in the UK then you would in America.
Americans may drive further on a road trip but that isn't a good enough explanation as to why the petrol/gas prices are cheaper...the simple fact is that the UK is taxed to hell...another example is the recent WiiU release, in America it's priced at $299 in the UK it's £299 which is roughly $475. Is it cheaper in the US because they have to drive further to get it I wonder?