it was orignally based on how big your vehicles was and how much wear and tear it was going ot put on the road

then, someone clever, decided that they could tax the emissions that might be produced by the vhicle, so that's when the tax bands came in

apparently the road tax and general motorist charges come in at around the 340 billion mark every year, enough to pay for plenty of policeman, nurses, firemen, schools, hospitals etc, but not quite enough left over to pay for the roads they aer actually supposed to be paying for

so go figure eh