Nota bene
why bother getting a French SIM any more when you can simply bring a Three UK SIM with you and use that?
If you need a French number, get a SIM from Toggle Mobile and forward your existing UK number to its default UK number and then set up a French number on it.
French SIMs can be both difficult and expensive to run.
In my experience SIMs do not reach their destination either sent from or sent to France. The French post office typically removes them in transit, just as it does cash. On the other hand, some of these online retailers may hold stock outside of Metropolitan France.
If and when I use a French SIM, I use SFR as I can get a gig for around 10€ for a week (which is outrageously expensive compared to Irish SIMs ). The only advantage is that you can use it to stream media, which you can't with the Three UK SIM while abroad. Even then, it's problematic. Shops don't always have the same term for the top up as SFR use -you must ask for "connecté partout" to get the correct top up as the exact same purchase won't top up a SIM for say, iPhone, but will for say iPad (internet mobile vs connecte partout). The other major issue then is to actually apply the code! You're supposed to be able to do it by SMS but typically this fails (probably because you haven't got any credit !) DOH!