A thought occurs to me on pollution emitted by cars. The gov almost certainly intends to tax on theoretical CO2 emission per mile, but it may be that pollution doesn't match that. If they tested a good many representative samples of cars and measured unburnt hydrocarbons, carbon monoxide, oxides of nitrogen and so on, that might not even nearly match fuel consumption.

For example, burn hotter to improve Carnot efficiency, and the result is to oxidise more nitrogen. Add a catalytic converter, and make the engine provide it with sufficient heat to work in UK winter, and 3% to 10%, (according to who you believe) more fuel is burnt.

Of course some of this is tested on the MOT, so cannot be worse than whatever figure is specified at least once a year. Still, if all the carbon tax aspect of things was on fuel - after all, there must be an exact correlation between fuel burned and CO2 emitted - and graduated road tax on pollution - that could be justified as an attempt to improve air quality, and it might even do some good in the long term.