I see them twice a week and have never even sat in their living room the whole time I’ve been with DP, it’s always a very formal cup of tea at the table
You are not even married to their son but you see them twice a week? Why are these people in your life at all? Even as a married couple I wouldn't expect to spend anything like that much time with in-laws, and that's if we got on with each other.
DP said they are just quiet, fair enough but after 3 years? ... I'm the least confrontational person ever.
Well no, your DP is even less confrontational than you are. To the point where he's barely a partner to you. He doesn't have your back, does he?
Originally the plan was to stay down here as the majority of jobs in our field are London based
Well stay down here, London is huge, you don't need to live 15 minutes from his awful parents.
DP has tried to defend me but they won’t have it.
Most men would have walked out and left them to it, until they apologised to you.
I think he wants to keep peace both sides.
He can't. And if he is not on your side, well, do you really want him?
DP’s mum is a mum of four , and I’d have loved her guidance and advice.
Really? What kind of "guidance" or "advice" could you expect from a woman who behaves the way she does? If you stay with him, then at best he cuts off from them and you are without in-laws. And bad as that is, it's probably the best that can happen. At worst you will have these awful people in you life, despising you and resenting you, forever.
I suspect your DP thinks he can avoid conflict with his parents by not marrying you. He needs to get real and the two of you either need to make your commitment clear by getting married (regardless of his parents approval) or split up. With a baby on the way it sounds like marriage. Either way your DH needs to grow up and cut the apron strings.