I think i would go, with some careful research and planning of course.
I think a poster up thread made a valid point that holidays with children are never as relaxing as holidays without children but if your OH is helpful and you are with friends then hopefully that would help the burden of cooking, washing, child wrangling etc in a strange location!
Another good point is the washing, sorting etc when you get back which is a particularly unenjoyable part of a holiday for me so you would need to be super organised with getting newborns clothes, bedding, muslins, food for freezer, UK hospital bag etc ready for when you get back.
I can totally appreciate the lure of a cheap holiday in the warm with some cheese and the odd half glass of wine, especially if you enjoy travel and holidaying abroad and it is, or has been big part of your life.
As for a potential birth, I would try and go the week early to give yourself a bit of extra breathing space. You could even have a "home" birth and have the baby in the holiday house.... Much more relaxing than in a French hospital if you don't speak French however nice they are! And then no worries about child care for your son or getting to the hospital etc! Then you would only need to have hospital care if there were a problem.
Anyway, I can totally understand your desire for a holiday, especially if funds are tight and the prospect of another appears to be years away. Good luck with whatever you decide and hope you manage to have some time to relax before the baby comes. I think visiting your friend in Paris might be a good compromise of that's the way it goes. Bon chance!