Alas, the government isn't happy with mere carbon taxes - it places superfluous and hence unnecessary additional burdens on us. It does this because politicians usually do not know the correct rate in a cost-benefit analysis between the externalities we emit and the freedoms we enjoy. Let me give you a personal example of this - the MOT emissions test. I drive a high-emission Subaru, and in order to enjoy this I am penalised with a higher rate of car tax. I also have a higher fuel bill than most. But both of these measures are fine by me in a free market where free-choice rules: I enjoy my Subaru and its fast-driving capacity so I soak up the additional cost. In a free society you may prefer something similar, or something very different, like, say, a more efficient Nissan Micra. You'll pay less than me in tax and fuel (per mile) but I'll beat you off the lights every time and probably get to my destination quicker than you. As long as we're both happy with that arrangement (and our choices indicate we are) then all is fine.
The trouble is, emissions testing at the MOT centre means that it doesn't stop there. Drivers cannot get an MOT certificate if their vehicle’s exhaust emissions are too high. This has come at a considerable extra cost to me (and other drivers like me). I had to pay £650 to replace a perfectly good sports-cat because it couldn't lower the emission levels to below the legal limit, and I will have to do this every few years for the same reason. Add to that the fuel costs and wear and tear each year getting the sports-cat hot enough to pass the test, and the fact that many other parts (oil, filter, lambda sensor, valves, etc) need changing more regularly to keep my emissions low enough, and this amounts to an unnecessary set of costs that I have to incur on top of the carbon tax I already pay to run a Subaru.
These costs are unnecessary for two reasons: firstly because they are going to have no significant bearing on future environmental changes at all. And secondly, a carbon tax (on my car tax and my fuel buying) already does the trick without imposing all the additional MOT emissions costs on me. If I find I want to lower my tax and insurance and reduce my fuel bill I can choose to sell the Subaru to a buyer who wants a fast car. Through that transaction we both win. If I can't sell it due to no one wanting such expensive car bills then it's a signal that my next car should be a more environmentally friendly car. In each case, autonomous decision-making rules. With the MOT emissions problem I have to fork out hundreds of pounds just to keep my car on the road. The government is already getting its pound of flesh through my increased road tax and fuel bills, it's unnecessary and hugely damaging to my bank balance to add MOT emissions expenditure to the mix.
There's also the unpredictability factor which gives us inopportune bills for which it is hard to save - and this affects not just high-emission drivers - all drivers have the same issue here. When your car tax and fuel bill is consistent with the kind of car you drive you can plan your year around it, knowing roughly what your expenditure is likely to be. If you end up with a part fault you can replace it knowing that the part needed replacing. But with these emission laws, bills can come in not through non-functionality of parts but through cars not being able to get through the MOT emissions test without having them replaced. As I and no doubt many drivers have found, this frequently brings about unexpected bills of hundreds of pounds that we need to pay, not to make the car roadworthy but to make it MOT emissions-worthy (a very different thing).
If we think it's reasonable to pay a bit more to run gas guzzlers, then it's true we need some mechanism to know whether people are running high-polluting vehicles or not. But we already have this mechanism in the form of higher fuel bills for gas guzzlers and higher car tax to account for those emissions, which, to me, renders the additional (and superfluous for reasons I indicated) MOT emissions test unnecessary. I'm not saying we don't need an MOT test at all - we just don't need the emissions part.