It is a shame to always have to take one's lead from the French, but I would have thought even more so in your case to be taking your lead from the Liberal Democrats. If you are so desperate to shore up the flagging revenues from your Congestion Charge, would it not have been a slightly more grown-up way of making policy in this area to set some proper rules so people can understand? For instance, would it not have been more sensible to introduce graded taxes for engine size as an environmental policy? Would it perhaps have been fairer and more straightforward to introduce graded taxes for size of car? The problem is that I think most will see this as merely some whimsical piece of policy-making because you personally do not particularly like those sort of cars.
The benefit of my suggestion would at least allow people to bear the rules in mind when choosing their car in future. For instance, people might think twice about a Jaguar or even a people-carrier (Tony Blair beware!) because those are big cars that take up considerable road space, and who knows where your whims will take you next. Equally, you might also want to consider taxing extra cars running on four-star leaded petrol because they can be smelly. Why not consider these approaches, because at least it would be fairer and more straightforward than a simply whimsical approach because you do not happen to like a particular type of car.
It is not all just meaningless grandstanding to grab a few cheap headlines because I understand that the borough with the largest number of these cars is Kingston. As far as I know, your Congestion Charge does not reach there. Unless you are intending to extend your Congestion Charge to Kingston, surely this is all just meaningless showing-off by you?