I don't know much about legal things or selling houses - but it sounds to me as though him saying that you might come out of the divorce with 300-359K is just his way of trying to scare you into not going.
From what you have told us your house is worth it may be true that you get a lot more than that. I would ask your lawyer just to check. Your x2B seems to be doing everything he can to try and regain control now that he sees everything might be slipping away.
It sounds like he behaved appallingly at your DS 2's b'day - what an idiot. If he can do that when his kids are around, especially when it's one of their b'days then he does not deserve to be able to spend time with them as and when he chooses. Get away from him and down to London as soon as you can. Do you have other options for working or is starting up a business your only choice?
Your ex2B lost any rights to decide what you do or don't do the day he walked out. Now because he sees that you are determined to go, he is scared and so is behaving childishly to try and scare you into not going
I can't pretend to understand what you are going through, but I would say get to London as fast as possible. If you don't then I can't help thinking that you would spend the rest of your life wondering "What if.....?" I know that probably sounds cliched, but I'm a firm believer in taking risks every now and again.
You are such a strong person that I'm positive you will land on your feet if you go to London. Moving away will be hard I'm sure, but I think it might help your healing process in the long run - after all,he won't be able to drop in and out of your life as he pleases, nor will he be able to control anything you do.
Please please please do what is best for you and your lovely boys. Sod him, don't even give a moment's consideration to his feelings. You are so strong Spook and you HAVE come such a long way
Phew, sorry bit long