I think too many people are projecting their own views on to the OP's situation and assuming a lot. As someone who comes from the Carribbean presumably she is able to "function" both in a city or with a quieter pace of life. She has already stated that her husband is willing to compromise with Cardiff or Bristol, and North/East Cardiff is a 20 minute drive from Abercarn so handy for babysitting and support so would make perfect sense.
Not all people have to be ambitious and driven, and want to work 70 hour weeks or be in senior roles. I got the sense that the OP's partner discussed being a SAHD for a couple of years in relation to what they would do for pre school childcare, not as a permanent fixture rendering him completely useless as a human being contributing to society.
My own husband isn't particularly ambitious (and grew up 30 minutes from central London FWIW!) but earns the average national wage and helps raise our children. He initially discussed being the one to stay at home when I had our first child but he wasn't cut out for it so I was the one who went part time in the end, and will stay part time until our youngest goes to school next year. I don't think he has ever accused me of being lazy and unambitious for making this decision though, but then I'm a woman I suppose it's acceptable.
Contrary to what's been repeated here, I and many other members of my family have never found difficulty finding jobs in this area. There are always factories, shops and restaurants looking for staff. Cardiff, Magor and Chepstow have lots of large distribution centres for Tesco, Aldi, Wilko's etc. not to mention call centres like Admiral who employ thousands, and new start ups like Monzo and Starling. These might not be the most high powered MN jobs but hey pay bills and are flexible. However, choosing to study part time and work at 30 doesn't shout unambitious to me anyway so I'm sure he could strive for more if that's what they both preferred.
Obviously this current situation could change workflow massively so it's all a bit up in the air anyway, but the OP did say that this was the very early stages of TTC and marriage planning so anything could happen in that time anyway.