Charging by the mile seems the fairest way.We clearly need to move to road user charging due to loss from duty on petrol and diesel. You can't put the same duty on electric, otherwise it would impact on your heating etc. The other alternatives are raise other taxes e.g. income tax or VAT or cut spending by the amount.
Out of those options, surely continuing to charge motorists based on how much they use their vehicle is the fairest? The difficulty comes in how you differentiate between the electric equivalents of the cheap supermini and the brand new V8 Range Rover. Currently mpg means the bigger vehicle pays more, but if it's purely a mileage charge it wouldn't matter
For it to work, keep it simple.
The more expensive and less efficient vehicles already pay additional tax, be it VAT when the vehicle is purchased or fuel duty VAT when the vehicle is used