I imagine, Gromit, that the reduced sales of large vehicles has far more to do with increased fuel prices than the desire of potential owners to �cut their emissions�.
Schemes such as the hydro-electric powered trams you describe are fine. They may produce a small benefit (though I doubt it) and there is no direct cost borne by individuals. But as I pointed out, the production of the electricity is small beer when considering the effect of electrically powered vehicles in total.
Like I said, even if the UK (and indeed the whole of the EU) reduced its emissions to zero (neither practical nor desirable unless you want to live in a cave in the dark and cold) the overall global effect would be negligible.
All this can be argued over ad infinitum but it somewhat evades my main point (which I think is being missed and that�s my fault), which is about the dishonesty of politicians. To justify punitive taxes under the banner of "saving the planet" is dishonest � and that�s where I part company with �conventional wisdom� and become a heretic.
To start with it has not been demonstrated that the planet needs such a rescue. Even if it does, excessively taxing people in the UK because of the car they drive, or because of the household rubbish they produce, or the number of flights they take will not see any significant global benefits. It is the modern day equivalent to Window Tax � devised to extract money from the population when other revenue streams have reached their practical limits.