Broomstick - I can assure you that anything serious notified to the DGAC does get pursued back to the UK. A few years ago I busted a TRA in France (in the days when the French IGN charts did not yet show their nuclear power station TRAs, and the ais.org.uk notam system did not yet carry the info either). I was under radar contact with a special squawk issued; French ATC said nothing at the time but asked me for the pilot name, his inside leg measurement, etc. 6 months later I heard from the CAA who were asked to prosecute on behalf of the DGAC. The radar trace (which I got a copy of) proved the special squawk and made everybody wonder why ATC did not tell me about it at the time but that is another story...
So I think if we did see the alleged huge numbers of Brit pilots flying illegally in France, there would be a lot of cases heard of over here of people getting done. The GA rumour mill would make absolutely sure of it. Yet, nothing, zero, zilch has been heard. I reckon that if some cases were suspected there was no proof and no action was taken.
It takes only a few cases of French police pick on an N-reg pilot and turn his plane over checking his VAT certificate and the words "instrument pilot" on his plastic card, for the word to get right around.