I'm sorry I was a bit flippant there - only your part time not-very-big salary is twice the most I earned at my peak! I kept 4 DC, a non-earning spouse and too many pets on it, but to be fair we didn't have a mortgage or rent at the time.
Ok, in London it's true £60k doesn't go far, mainly because of the price of property which is absolutely ridiculous (I used to live in reasonable commuting distance instead, which made a big difference). However, I would have thought your extremely high earning H (STBXH?) should continue to contribute to the mortgage while the children are under 18 at least, because it's still his property even if he doesn't live there, still an investment for him which he will be able to realise when it is eventually sold. Which is fair enough. He would in any case have to pay maintenance for the DC, though I suspect not for you as you have reasonable earning potential. It should suffice. Meanwhile he's not exactly going to be confined to a grotty bedsit on that income.
As usual in such situations, I recommend good (not the same as expensive) legal advice so you know where you stand, before making any decisions. With any luck, knowing you aren't afraid to leave may concentrate his mind. Up to now he seems to be regarding your complaints like an annoying whine from a domestic appliance - just oil it occasionally but don't worry as long as it keeps working. You're worth more than that.
To be fair to your H, it must be massively hard to know, at the back of one's mind, that one is less loved than one's sibling(s); perhaps impossible to admit even to himself. If the end result is to be that he expects you to accept bad treatment from his family, though, I'm afraid understanding why he may feel that way is not enough.