I posted earlier OP, but confess I'd skimmed the thread. Your updates get worse and worse every time.
Please read GentleParent's post at about 10.13am this morning. Absolutely spot on.
So many red flags, I'm worried for you.
- MIL saying you're being selfish by not moving into her home in her village! Unbelievable!!! Who thinks that????!!
-DP supporting this view! Massive concern.
-DP living at home with Mum until 29, and after only 3 years wants to go back to living with Mum 

-You quitting your highly paid job and taking a huge pay cut to please him. Again a massive concern.
- DP taking 5 years to do an OU degree in an industry where only those at cutting edge progress. Even on a FT degree the hours are not actually FT and he could work. What a dubious decision on his part. It feels like he sought the easy option- sorry to say OP.
- him suggesting he becomes a SAHD- even though he won't have got a job by then; this will only set his "career" back even further and basically he'll be living off you forever.
- DP hasn't made an effort to make friends in London, eg joined any sports teams etc feels like he hasn't committed to giving it a go in London- he's just biding his time until he can drag you back up to his village.
- DP unwilling to consider a compromise like Bristol / Cardiff - very dubious
- saying he "will stay if he has to but won't be happy about it" is a very emotionally manipulative way to phrase this.
-you saying he treats you like a Princess. I'm always dubious of that one. Never a good sign ime. You seem to contribute the lions share of everything and he has asked you to give up a lot. I'm worried there's an element of control. Not saying he's a horrible person, but given his mother's attitude perhaps the apple hasn't fallen far from the tree.
I'm sorry if it's upsetting and don't mean to sound unkind. You sound amazing, gutsy and you've made a real success of yourself. I'm not sure what he's bringing.
Have you actually properly investigated if you can afford to buy say, using Help to Buy in London? I'd have though you could. It would be worth properly investigating this. I'd try to buy by yourself, and have a clause which keeps him off the deeds to be honest, until he can contribute his fair share, but I feel a growing sense of mistrust about him and protective over you having seen your updates.