A car starts with the base price, then special car tax (introduced in the 70s I seem to remember) is added, then vat on that and any extras like plates and delivery. A van does not have the special car tax. If you later add side windows, it becomes liable for special car tax, calculated on its value at the time the windows were added, plus vat on the car tax! So beware if you buy a used van privately if windows have been added. Get proof the car tax has been paid, or you, the current owner will become liable when the revenue catch up. They don't chase the previous owner.
Think of it as your contribution to digging this country out of the hole we didn't put it in.
Didn't they abolish car tax some years back?