Whether it's likely, I don't know. There are no doubt thousands going back and forth all the time. But let us stop for a moment and consider the 'problem', ergo; a socialist government in France will whack up taxes, leading to some people moving their businesses or affairs over here, or, indirectly relocating in the search of jobs, which will be fewer and further in between under the burden of a socialist government. If such a thing is really a problem, the solution would be to whack up our taxes and scare them off. But ... wait a sec, something's wrong with that logic.
As I said above, I don't know if Duncan Smith is being spun, but surely we should be trying to make our tax regime competitive in comparison to other countries, by competitive I mean less monstrously greedy than it currently is. We should be, if we were true to the principles which made this nation great, quite content to see businesses relocating here from the continent. Indeed, if we could ever escape the chains of Brussels wound round our necks by our treacherous mandarin class and their political lackeys, this is exactly what we would expect to happen.
I say Laissez faire. Cut taxes, turn the regulations into fuel pellets, and we can run the power stations for a decade, and make this island a place to do business again, and if that encourages some enterprising Frenchies over, then bienvenue.