It’s anyone’s guess of course, but I find it very hard to imagine that 1 in 10 Brits living here is in an irregular situation. A lot are married to French people so they’re OK, a high proportion are workers so they’re OK, many are UK state pensioners and they will pretty much all have their S1s so they’re OK, so it’s really only inactifs who would potentially be irregular and they’re a small proportion. If out of that group you discount the foot in both camps brigade, which I think you can because they still have rights in the UK so it simply means they’ll spend less time in France in future, and from what you read on other forums a lot are already working out their plan B’s, I would be surprised if as many of 1% of total UK immigrants in France have cut all ties with the UK but not regularised themselves in France. I may be wildly wrong of course.
To me, France puts so much emphasis on droits et devoirs in all of its dealing between state and citizens that I find it a bit hard to see it setting a precedent and dishing out rights to folk who haven’t met their obligations. I’m sure it will allow a generous window for obtaining whatever residence document is required, but I think it will be pretty strict in applying the cut off date, be it next March or 2020 or whenever. And I don’t see it kicking people out - when a cds application is rejected there are two possible outcomes, either you get a simple refusal letter or you get a refusal and an instruction to leave. What I feel is most likely to happen is that if the conditions aren’t met it won’t issue a cds but it won’t issue the instruction to leave. So in effect they will become a sub-category of “sans papiers” which is already a kind of official unofficial status where people who are here irregularly are tolerated but have minimal rights.
But for purely selfish reasons I’m still hoping for no Brexit, and it’s looking more of a possibility by the day.