Both of which, although mainly the first would be pretty good idea, if far more people did it no?
Its got a cost as well of course, plus like anything to try to avoid tax, that the rules can change.
As we know as well, it would be pretty unlikely someone in the UK would manage to have so much solar that during a substantial part of the year they would not generate enough anyway.
And if they did so low mileage that they were able to charge on the solar panels they would probably be better to just pay the premium on those few units