I agree with much of your thoughts Jon but have to agree to disagree on this. You have hit the nail on the head. It is entirely and rightly fair that a vehicle that pollutes the atmosphere more should pay more...why is it not fair?
Flip it the other way. Is it fair that someone that drives a more efficient car that pollutes less should pay the same amount as someone with a bigger or less efficient or more polluting car? The current system levied on C02 bands is there for that exact reason.
I'm coming from an angle of an owner of a couple of bigger engined more polluting vehicles. I'm more than happy to pay for my fair dues of car tax. I'm just not overly enamoured with paying for a tax on a car that is not on the public highway or being used.
Weirdly we pay a fixed rate of £265 car tax on the Outlander PHEV per year as it is classed as a commercial with only 2 seats. It is lighter than its 5 or 7 seats non-commercial equivalent but an otherwise identical vehicle. The non-commercial version attracts zero car tax as is less than 50g/km C02. Work that one out?!