The excuse is that it's to do with carbon dioxide emissions (as distinct from genuine pollutants which, as you say, are checked at MoT time). It feeds from the fantasy science that predicts the imminent destruction of life on earth and ignores the fact that cars (and these cars in question, especially) produce a very small percentage of total carbon dioxide. It makes a good excuse to increase taxes, apparently.
It seems possible that with the threat of a backbench Labour revolt, and with the weakest chancellor in British history, the proposed taxes could be changed. But the government is so short of money, that this will be difficult to achieve.
British vehicle users now pay one billion pounds a week in taxes. The government is planning to take 60 per cent of next year's additional revenue in motoring taxes (reported in Daily Telegraph Motoring, 12.7.2008).
If implemented, this will bring real pain for less well off motorists; either they pay the extra tax, or they trade in their cars (now much reduced in value) for vehicles too small for their needs, with secondhand prices inflated by the chancellor's distortion of the market.
Good, isn't it?