I can see the rationale behind it, at least.
I'd like to hear that.
I do agree that increased tax on fuel is a perfectly valid alternative. I'm not sure that it would act as such a powerful incentive to alter buyer behaviour, though. I'm assuming that the government want to dissuade people from buying the most environmentally-unfriendly cars.
For most people, I think a Ł300 lump sum every year (i.e. road tax) is a greater disincentive than another 4p on the price of a litre of fuel. Which, after all, is going up for the foreseeable anyway.