I haven't read all the responses, sorry.
Your bf's argument about a flight being bad for the baby is not really valid; you'll likely be walking about and driving places even at 9 months, and a flight isn't especially stressful in comparison.
On the other hand, unless he will also be going to the UK a couple of months before the birth, you're cutting him out from the experience of being with you, watching the bump grow and getting ready to be parents. And what if you had the baby suddenly and he wasn't there?
I did see a couple mentioning the sorry state of the NHS and would agree with that. Have a look into how long you would be likely to stay in hospital if you give birth there - don't know how it is in France, but here in Germany you don't just get sent home hours after the birth.
On the other hand, if you are quite anti-French (or at least anti- not being in the UK) then that sentiment might leak over into your experience of giving birth, and make it less pleasant than a comparable birth in the UK.
I gave birth abroad and my mother came over immediately after, and was quite helpful. But I would not have wanted her present for the actual birth; is that what you want?
You don't have to have everyone over to visit at once. If you're homesick, it would be nice for you to have people coming over every month or two, seeing the baby at different stages of development, rather than just as a newborn (they're cute, but basically just sleep!). You would then have something to look forward to every month.
You do need to get married. It is nothing to do with romance. You need the full legal backing. You're having a baby with someone who has totally different wishes for the future than you. That was already a very risky thing to do; you could well end up unwillingly abroad as a single mum, or your child might end up unwillingly growing up in the UK with no dad about. Don't add a total lack of legal support to that already risky choice.
Before I had children, my ex made vague noises about how we might go to live in the UK one day, but in the end, he was the main earner, and considered that that gave him the final say. I would not bet on you being in the UK for at least the next 18 years or so.
Having said that, I wasn't that keen on the idea of staying abroad at first, but now I'm happy here and have no plans to live in the UK again. A year and a half is no time at all. If you want to feel more at home, you need to make efforts to find local friends, and talk to them in French so that you get really good and can be yourself with them. It's possible, if you make an effort.