Unless you happen upon the scant portion (~600 LPs) I brought w/ me when I moved here, don't expect to find great "hidden gem" rekkids (anywhere) in France. The musical taste en general just ain't been here in many many decades. No ****.
As for London, there are good record stores, one of which is owned/operated by Nick Saloman, he of The Bevis Frond fame (writer and geetarist), but I don't recall the exact name of his shoppe. It might be the same as his refcord label, i.e., Woronzow Records. Dig about The Web a bit and you'll likely find it/him/them/etc.
Past that I have long-time fellow record trading friends in/around London, but they haven't shoppes as such and the rekkids they purvey are indeed (often) treasures that will cost you A LOTTA BRITISH POUND STERLING -- equal to hundreds (if not thousands) of U$D, thus I'll not pen a list of those here. Best you deal w/ them via email and/or via their websites as they are not keen on inviting strangers into their respective lairs, understandably so ciz y'never know...
Yep, hit the web, look for Nick's joint, and while there look at other in London possibilities. And you can check for shoppes propre dans Paris but be prepared (again) for HIGH asking prices, cuz ain't nuthin' come cheap in Paris, my friend. I have found the best (inventory and cost) rekkid stores in France to be 3-4 that are situated in the city of Nancy, but that's a 1-hour (maybe longer?) TGV ride from Paris. Long way to go to browse used rekkids, but that's merely my opinion; perhaps for you it might be worth it. Hell, just visiting Nancy at all, going to Centre Ville, and standing @ Stanislas Plaza (Place) is in iteself worth the trip to Nancy. Couplpe of nice audio gear shoppes also in the vicinity of Nancy's centre ville.
Bonne chance.