Fuel tax in the UK is the highest in Europe. The road infrastructure is among the worst, mainly because very little of the tax is actually spent on improving the road system. If you need an example of this, take a look at the A303, the main trunk road from London to the Southwest. It will probably never be finished.
So here is a useful graph from this site, albeit a little out of date:
If you do the math, tax is around 65%. Now that is bad enough, but remember, for many people fuel is paid with after-tax net earnings.
The excuse, of course, is the aim to protect the environment. A reasonable excuse do I hear you say? Not really, because the alternative transport solutions simply do not exist for many people.