But anyway, Why on earth should a Ferrari be taxed less than a VW UP! Just because one might be driven more?
Why not? The more you drive on the roads the more you should contribute and the more you 'pollute'. Either 'road tax' is designed to encourage efficient driving, in which case it makes no sense to tax a car that does 500 miles a year many times more than one doing 50,000 a year, or it isn't, in which case why is the road tax for a £30,000 520d only £30 a year when the road tax on a £20,000 Kia People carrier is rather more?
Putting it on fuel acheives all goals - the more fuel people use the more CO2 the emit and generally the further they drive, so taxing fuel instead means the tax people pay is proportional to the use of the roads/pollution of the environment.
I have two cars, one is hardly used, one is used all the time. The one that is used all the time is bigger, heavier, more expensive and... £100 a year cheaper to tax

