As some posters have said above, there are some major red flags here, OP...
You sound energetic, ambitious, forward looking and focussed. From what you say, his mindset is very different: he seems to have a much more narrow, insular and restricted attitude to life. The fact that his mother is weighing in and trying to pull him back to the small town he was born in is worrying - not only because it's an indication of her future role in your relationship, but also because it suggests that this small town, limited mindset is really ingrained in his family and his community.
You moved continents and made it work. You built an impressive career. Meanwhile, it took him until his early 30s (and with your input) to even think about moving out of his mother's house and expanding his horizons beyond the easiest local admin job he could find. Now - after just a few years - you have already sacrificed a chunk of that for him. There's a real danger that this trend will just continue.
I would think carefully about the environment you want to raise children in. Your boyfriend blames his parents for "not pushing him" (a bit of a cop out by the time you're in your 30s, but OK...) - do you want your kids to be surrounded by those attitudes? Some areas of Wales are severely economically depressed - unemployment has been so high for so long that there are generations who have never had many opportunities and it becomes the norm... Expectations are low and it takes a LOT to overcome that. Of course, it's not like that everywhere - and it's not a problem that's unique to Wales - but it certainly sounds like your boyfriend and his family are in that mindset.
That's not to say that you can't be a good match - there are plenty of couples where one is ambitious and energetic and the other is more in their slipstream. But that only really works if he's willing to be supportive of you while you go out as the main breadwinner - and it sounds like that means staying in London. Could he get on board with a version of life where he is the stay at home parent and supports you in your career? So you can level up again at work if you want to, pull the occasional late night at the office when you need to and know that you have the support at home? Could that be an option?
Otherwise, there's a very clear path here that leads to you being trapped with young children in a tiny, less-than-diverse village in Wales with a community you feel excluded from, a job that doesn't challenge you, a husband who refuses to challenge himself and a mother in law running the show. Be careful.