I love my mum dearly but she can be a complete nutter in terms of emotional blackmail. I decided once I'd left for uni that I'd never live with her again - that was over 20 yrs ago and and we get on much better for it. Like Naat, I'd definitely rather be in my own space, supported by friends and other family, than stuck with my ma on her territory for 3 months!
If you're doing antenatal classes, those first weeks are when you'll be getting together with other new mums some of whom will be a real lifeline for you in the months and years ahead - as they are for all new mums, with or without a DP - and it sounds like your brothers won't let you down.
If you decide to stay in London and your mum does dig her heels in - or if it's simply difficult for her to leave her home for any length of time - is there a close friend you could think of as a birth partner? That's the only part of the process where you don't want to be alone and it would be hard for your mum to get there at short notice. You could have a rota of friends staying for the first few days - but I bet your mum would be down like a shot anyway!
Some women seem to have much more mutually dependent relationships with their mums than I do, but to be honest I don't envy them: I'm happier facing life's big moments as an independent grown-up, and in some ways feel that my mother, for all her oddities, has equipped me to do that.
If you go ahead and birth in London, you can go up north on your terms for a shorter stay and don't forget that your life, job, home, friends and support network are not there.
Finally, if you can announce your decision to your mum in as controlled and non-confrontational way as possible, without linking it to her recent nasty comments, that would make it less likely to boil up into an argument. I'm sure she cares about you a great deal but she has to learn that you're about to be a mum in your own right and she has to let go and treat you as an adult, not a naughty child. You and she will get through this; it's not disastrous and terminal, just part of how she has to learn to be grandma while you become a mum.
Of course it's up to you, that's just my take on it, but whatever you decide you can lay out the ground rules very carefully and get her agreement.
Cheer up chicken!