As I understand it, the British government has jacked the price of diesel up above petrol to discourage people from buying diesel vehicles. They are dirty, and they pollute the air much worse than petrol burners.
They also collect more taxes on the higher priced fuel.I guess that helps to pay for the lung problems that the diesel fuel creates. Someone has to pay for that "free" national health care you enjoy. :-)

In my part of the world, there are scores of fuel stations all competing for my money, so a quick check on the computer will tell me where the lowest fuel price is to be found, and that's where I go.
Price fixing is illegal here, so they either compete or go under.