Used cars do cost a little more in France it’s true (although exactly how much more, in real terms, obviously depends on the exchange rate on the day you buy it). I guess it depends on how much hassle it would be to get it conforme and registered, and also the expense of changing the headamps if you can’t find replacements on eBay, and whether you really feel that spending your time in France sorting all that out is worth the saving you make. There’s also the issue of which side the steering wheel is on, personally I don’t consider that to be an issue but I know some folk do.
Leaving it on UK plates isn’t really an option because of the insurance. French insurers won’t insure a car that you have no intention of immatriculating, and UK insurers won’t insure cars that are kept outside the UK. Giving your insurers a UK postcode where you say the car is normally parked overnight, when in fact it’s not left there overnight from one end of the year to the next, would not be a good idea.
I have to confess I’m finding it a bit hard to get my head round buying a holiday home in France and then baulking at French second hand cars prices, ha’porth of tar comes to mind, but one valuable thing about this forum is it helps one to appreciate that we all see things differently.