Ok, Vehicle Tax. But "Road Tax" means prodding fewer letters on a tablet...
As for taxing road use for electric vehicles, a "simple" way might be for all cars to have to attend an MOT test, not just those over 3 years old. Up to and including the third year, this would merely consist of tyre checking and recording the mileage, which would then be forwarded to the DVLA who would then issue a tax demand for distance covered.
Fun all round!