When all UK homes have "smart" meters they are likely to have a function that can tell if a car is connected to a charger. I think if you take the grant for a home charger you have to have a smart meter. To tax public chargers would be relatively easy.
I suspect smart meters will eventually be used to charge the end user a variable tariff for juice, which will be constantly variable throughout the day / night depending on demand
There are also intelligent charge points which can feed power back into the grid from EV's that are plugged in with spare capacity