the problem is that if governments tax petrol on a percentage basis

Only the VAT element is taxed pro-rata. The fuel duty is fixed, currently at 50.35p/l. In fact taxes on fuel are a lower percentage than at any time in the last 10 years at least.

The local supermarket's current price (unleaded) of 115.9p/l works out as:

17.26p VAT
50.35p duty
48.3p fuel

Or about 58.3% tax. The 2p rise in duty would translate at current fuel prices into a 2.4p rise at the pumps.

In real terms, petrol is about 24% more expensive in real terms than in 1983 , having been cheaper for much of the time, and having risen nearly 30% in the last few months- see the link for the rise since March. Total taxes have been over 50% most years since 1952 (pdf).

The fuel protestors are protesting to the wrong people- it is world oil prices, driven by economic growth in China and India, by speculation, and by the increasing cost of extracting oil, that has caused the price rises.