Thanks for the info Danscowpie. Yes it's for my wife, we both live in France (I'm English, she's French) but for my career it would be much better to be in the UK so we have been pondering the possibility.
Pretty much what I imagined really re NATS. I think she would tolerate a conversion course but a full blown retrain is out. But maybe we should give it a try anyway and see what they say. Be interesting to know what the working hours/salary are/is like for UK area controllers compared to France...