I see.
I might as well just stop paying for the tax disc then, what a con, charging the same tax twice
At best you could call it wealth redistribution. Anyone who doesn't buy a category A tax vehicle clearly has more blood that can be extracted from their stone.
But they could have that money extracted from them by a higher fuel duty amount instead, you say? Not really. The headline price of fuel is too politically sensitive. People are historically used to paying for a tax disk. No one has yet protested over it.
So why not by weight as suggested in the first post? Indeed when I worked for the county council on highway maintenance all calculations were based on vehicle weight, and higher weight meant more damage done [so lots of miles in low weight lorry might not exceed that of a smaller mileage of a heavier vehicle].
Don't know the answer to this. Perhaps its too much effort for them to change.