Although any Government will squeeze you till the pips squeak, one reason is the chemical process involved in distilling crude oil.
This does not happen by some miracle, but by clever chemical process which , I am afraid, does not just produce pure petroleum for our car at 95 octane.
All sorts of other goodies come from the process, plastic, lpg ,heavy oil, light oil. So there is a ratio of oils produced due to chemical process. Now if demand for an oil that is in the process goes up, you cannot make more, not without all the other items, so the price goes up.
Diesel users are 'hoist by their own petard'.
Never buy anything for a 'tax ' reason, as chanellors will kick you in the teeth soon afterward- see lpg and how the tax is going up on that.
( ps, I would leave diesel to dumper trucks, it is not a nice substance with which to run a fast passenger car)