What if your brand new shiny Russian girlfriend comes over on an airline from Russia. Obviously you can fly her around the UK, but can you fly her GA all around Europe?
Russian citizens need visa for both the UK and Schengen. If she gets a UK visa, then she will not be able to travel outside the UK with that visa. If she also has a Schengen visa, then yes.
There really isn't any difference between taking the car and a GA airplane. That "visa waiver only on commercial flight" is a purely American thing.