Quote ... "You would be allowed to drive as Aston Martin – but only if it did 50-60mpg.”
It's not the inefficiency of the car per se that's the problem, it's the inefficient USE that some owners make of their cars. An Aston Martin that does 15 mpg but only travels 1000 miles per year consumes less resources and emits less CO2 than an average family sized car or executive car that travels anywhere between 15,000 to 30,000 miles per year.
If vehicle owners had a carbon allowance (which is some product of distance and specific CO2 output), then you can choose any car you want, but in so doing you may restrict the amount of miles you are able to do. This would force high mileage drivers to downsize to smaller more fuel efficient vehcles, and not penalise those people who choose to buy a larger or more powerful vehicle and do a low mileage. The system would be fair to the majority of motorists who purchase smallish vehicles and do average mileages.
The present system of fuel duty and road fund licensing is a mockery because it allows more affluent people to simply buy their way out of any environmental responsibility. If they had a carbon allowance, which could NOT be traded, they would have to think twice about the type of car they had to use. Once you use your allowance that's it. Enforcement would be quite simple - check mileage once per year when the vehicle has to be taxed or tested.