quite simple really
in france you can't get your bottles filled
in italy and greece I have never had a problem , always someone who will fill a foreign bottle
therefore you want a french bottle , and energas is the one that is widely available and will fit a locker that takes a 6 Kg calor bottle if that is what you have , or you can have a look to see if yours is the same size
énergaz les nouvelles bouteilles butane & propane
as you can see the website lists every point of sale in france under.. ou trouver energas..... divided by departements , and showing whether or not they stock both butane and propane
presuming you have the regulator screwed onto the bottle the regulator that fits the calor 4.5 Kg butane dumpy , 27 or 30mbar will do nicely ...all new fitments in europe for both butane and propane are now 30mbar but all the old 27/30/37/50 work fine ; of course you can get a regulator anywhere in france , most of the supermarkets sell them as they do gas
at that time of year I would get a butane because it is cheaper , butane and propane bottles are identical so you can swop between them if required
you are unlucky , the price of the bottle has gone up...from €1 !
as you can see there is a stockist in calais , just inland from the port , so you can stock up immediately you arrive [ if that is your route ] and save your UK bottle as a reserve for when the energas runs out , so come with a full one !
incidentally , energas is the cheapest gas available in france , only fractionally more expensive than buying from the lpg pump