I'm the one who used to live in Montpellier :)
I can't really help on the language front as my DC are bilingual anyway and we moved to France when the oldest was 18 months (and left when he was nearly 6).
As far as cities/places to live go, we loved Montpellier and found the people comparatively friendly (caveat to this is that I don't find people in France particularly friendly at the start, it does take quite a while to get to know them) and down-to-earth, and it seemed a much less snobby place than Nice, for example. As I say, it took a while to make friends but we did make some very good friends there. I used to go to an English-speaking playgroup, but generally didn't feel there were all that many English-speaking people around.
The school week is very different to the UK - for instance in maternelle/primary they currently do Mon, Tues, Thurs, Fri from 8.45 to 16.45 with a 2-hour break for lunch (they can either go home or eat at the canteen), with Wednesdays off. This is about to change though, either this Sept or next year depending on the place -- I'm a bit hazy about the details as it doesn't really concern us any more.
It means there isn't really time for playdates etc after school as it is already very late. Children generally do activities on Wednesdays and there may be scope for playdates then too. Though we often found that a few local mums would end up in the park after school on the way home, so it just depends.
My son went to a private Catholic school (not v expensive, about 40 euros a month) with maternelle and primary. I had many reservations at the beginning about school in France starting at age 3 and being very structured and rigid, but I was very impressed by the school and the teachers. V different to my own experiences though - they lock the gate at 9am and parents aren't really welcome in the school, but that's just France. There were no other English-speaking children.
Good luck with working it out and do ask if you want any more info about Montpellier. One other piece of advice - I would do some research (or ask here) about the differences between the French and UK education systems as it can be a big shock. It's much less child-centred and really quite rigid and hierarchical, with a big focus on things which I regard as unimportant. It has its good points too, of course, but it's important to know what you're letting yourself in for.