I would totally avoid Hampstead / Belsize Park
It's diverse, but the diversity sticks to their own - there's very little mixing between groups and there's very little social / community activities. If you aren't born and bred "in crowd" and brining in a big salary it's really not worth it.
The Northern line is regularly shit too - feel free to DM me if you want more info, we lived there for five years and moved last year. Much happier where we are now, even though it's a "less desirable area". Very awesome community spirit and everyone helps us out.
We were expats (I am not sure how long we can call ourselves that for) but deliberately looked for a more "non expat" community so we could create long-last friendships. It has definitely worked.
I would look either towards Islington, De Beauvoir, Chiswick, Walthamstow, Hackney. Some parts of Tottenham are really up and coming with people who've moved further out for affordability too - although we didn't actually look there.
Have a wonder a round a few places - looks for a variety of shops (Turkish, Polish, Chinese), check out the deliveroo options - lots of pop ups from home now are giving you a good feel of how diverse the areas are (and also prices), do the commute!