There are considerable refining costs and transport costs from the refinery to your tank. Especially so because in the UK I don't believe you have considerable refining industry so I assume much of it is imported and then FOREX comes into play. Fuel taxes, at least in the US, are constant per volume and not relative to price and in California make up currently about 15% of the cost.

On top of all of those factors I would only guess that at high prices profit margins fell by ratio which was recovered (or just increased) when prices fell.