If you really want to understand petrol prices and taxation you could look at this:
http://www.ifs.org.uk/bns/bn55.pdf
It's the institute of fiscal studies - they conclude
Petrol prices are now at their highest level since the fuel price protests of late
2000. However, the real duty rates on diesel and unleaded petrol have not risen
since then; in real terms, duty rates are at their lowest levels for seven years and
look set to continue falling, at least whilst oil prices remain high.
They include vat and inflation in their calculations and they do point out that our fuel is the most expensive in europe