diesel used to be cheaper than petrol until they decided to tax companies on the waste when petrol is being produced,diesel is a "WASTE"product and is taxed again. 
And as to taxes, the UK government needs about £1t a year to keep all these services that everyone is up in arms over due to austerity. And currently has about £160b a year shortfall.
This shortfall, without taking into acount unemployment/benefit strains, could be covered by permenently closing the NHS, the Police, and schools.
Too drastic? OK, what about the social security in its entirerity?
Still too drastic? Well, we're running out of easy solutions. Could close all prisons, but that won't save a great deal. Stop all road building/repairs. Nah, still not much savings.
We could stop HS2m that'll save somewhere between £40b and £100b over 10/30yrs. Still won't cover our annual overspend.
In a nutshell, the government needs to drag in as much as it can AND cut vital services. Doesn't need a rocket scientist to see that. So, they could cut fuel duty, but will still need to get that money out of us somehow
