To give a fairer and more efficient way of collecting a tax.
There are many vehicles on the road without a valid tax disc(estimated to be as many as one million). Could the annual charge be replaced by a charge on petrol which would have to be paid by all(one cannot avoid purchasing petrol). This could be easier and less costly to collect. The tax disc could be replaced by a disc issued by the insurance companies(some counties already do this) which would help to ensure vehicles are insured. The more one uses the roads the more the charge which would also be fairer.