It's just a complete load of BS in my opinion. Its always been the government's goal to tax the more wealthy people higher amounts than the less fortunate people, and as it stands they are using C02 outputs as thier gauge of this - there are just so many flaws in using these figures as a unit measure of car/engine size. I suppose the problem is that there are too many variables these days to determine the overall environmental effect each car has by looking at just one value.
If they were truely interested in the environment, you could suggest it would be more accurate to base taxation on the overall impact of each actual car based on individual readings taken from information on an MOT. For example, levels of Hydrocarbons, Carbon monoxide, C02 emmissions and soot deposits could be monitored to give a much more accurate indication on an engine's health, and more importantly, the threat it has to us and our atmosphere. What really irritates me is the fact that although diesels emit far lower levels of C02, they also produce far more toxic fumes than petrols - yet they still qualify for cheaper tax. Seems backwards if you ask me.
http://www.mflohc.mb.ca/fact_sheets_fol ... xhaust.pdf
The fact remains that Mr.Brown and his European cronies don't like us driving around in 15yr-old 3+Ltr saloons and SUVs. They want us to drive newer, smaller and ultimately slower yet more complex and potentially unreliable hybrids, while reserving the big Mercs and BMWs for those who say 'Tally ho' and 'Spiffing, my old chap' while wearing clothing made from tweed. It just begs a big question, which is what is going to happen to the taxation system when we all suddenly buy into these pieces of crap? They'll just jump on the next passing bandwagon and try and use that as a reason to charge everyone more money again.