The only reason the VED bands impact people's choice of car is because it is ALSO used to calculate the tax liability for company car drivers. No-one running a £40k car is going to give half a shite about a £400 a year VED bill.
I agree that people should have incentives to buy more efficient cars, but really the best way of doing that is by taxing fuel more heavily (yes!).
Good on Bojo for penalising those with older diesels and the particulate emissions they produce with the London congestion charge, for example.
Neither are those people going to be bothered about a hike in fuel costs...
Taxing fuel more heavily just hits the less well off, unless, that is, you want to return to the early days of motoring where the only people who could afford to drive were the rich?
Boris's "idea" is flawed in so many ways, it's just a sop to some of his voters... But just to give two examples, it "hits" the wrong people as the ones causing most pollution are taxis and HGVs, those are the ones he should go for as they far outnumber the odd old diesel car that makes it's way into the centre of London. Secondly "bad" emissions vary from year to year dependent on what flavour the Gov is and what the latest "research" is, there is no definitive answer to what is actually a "polluting car"... I have followed far too many smoking petrol cars to actually believe that older diesels are the only culprits... A car does not keep it's emissions low on it's own, it needs regular servicing, something many people don't bother with...
Personally I prefer the German method where they give you a sticker (red, amber and green) dependent on how your car does on emissions tests (or if new on the published data), the only issue with it is that it will not suit the driver of older vehicles as they'd be banned from most town centres and motorways...