I've thought about the problem of people avoiding paying tax, and thus bumping everyone elses up, a few times, and reckon the best bet would be to introduce a registration system, where by you need to prove you have a Tax, MOT, insurance, and are then entitled to a card that is similar to a credit card style swipe card, registered to the car. Without this card you wouldn't be entitled to draw fuel from a garage -
No tax/MOT/Insurance = No Card = No Fuel = stationary Car.
Simple idea, effective results