Congratulations on having twins!
I am in France, too, and have had two babies here. I concur with what Weta says. Not only do the medical teams expect you to want an epidural (I wouldn't go so far as to say they pressure you to have one, but it is def expected) but also, there's no alternative pain relief. No gas and air, no TENS machines, no Pethidine (is that still available in the UK? No idea). Plus, they pretty much expect you to be hooked up to an IV as soon as tyu get to the maternity unit, so even movement is severely restricted.
Hence, most women do want an epidural here!
There are so few options in France that the "birth plan" does not even exist. Why would it? You are not (as a general rule) in charge of your choices. There aren't any....unless you choose your maternity unit very, very carefully.
However, if you already have une sagefemme libérale you will be very well advised by her. She will know the system and be able to guide you. Just don't expect to be writing a birth plan including birthing pools and meditation techniques :)
You will already have discovered that the whole process tends to be more medicalised than in the UK. You can't really escape this, especially if you are haing twins. So my best advice is: embrace it! Remember than thousands of French women have babies every year, they have very, very high standards of care and very, very good outcomes in terms of maternal / infant mortality. They have very different ways of achieveing those great results, but trying to buck against the system which is in place can be tiring and unproductive.
Be guided by your midwife and don't read any too many UK-based pregnancy manuals as you will be out of sync with pretty much everything in France, even scans take place at different times in France (22 weeks as opposed to 20 week).
Good luck.