Ignore the word 'tax', should be about saving the planet moving forward as we're led to believe we're helping to do by driving an EV , hence why I think going on weight which is on the V5 I think, is a good idea. Will also help move people away from the idea of wanting a vehicle with the largest battery they can have for 90% of the time pootling around town (and carting that excessive weight around).
I remember owning a Z4 M Couple. A high emission car and was over £500 a year road tax. It's relevant as the car was only £40k or so(but 23MPG). Should maintain a similar approach for EV's. A heavy EV much more pollution from manufacture through to on the road efficiency.
It's another bad government decision, even moving away from it being exhaust emissions based IMO.
A flat fee for owning a vehicle doesn't seem right for these times either.