a raft of very bitter posters intent on telling me that sahps should get basically nothing
I think the issue with this particular scenario is not that anybody is saying OP should get nothing but that she seems to be expecting her Ex-H to get nothing and to have to go from living in a nearly mortgage-free house to starting renting at the age of 50, quite possibly for the rest of his life.
As far as the pensions go, that money is locked away for probably at least 10-15 years and, even then, cannot be taken as a lump sum without heavy tax penalties (and if it were, what would they live off on a daily basis?). It wouldn't necessarily be unreasonable for their two pension pots to be split equally and for them to get exactly half each - but that makes no difference to either of them for a decade or more.
Therefore, with the pensions out of the mix, the combined assets are worth £540K - and OP is wanting to take all of that, as well him give her/commit to pay an additional £10K on top of that.
Before retirement, this means that she gets close on 102% of their joint assets and he gets -2%. £85K is a very nice salary, but without a deposit (and starting off with £10K debt), he won't be getting a mortgage any time soon, if at all, especially not at 50.
This would also put paid to any intentions he does or doesn't have of being responsible for the children 50/50: he won't have room to be able to do so if he's only able to rent a small flat, so OP would then become the full-time (or close on) RP; meaning that, because he wasn't left with any money for a deposit on a small property for himself, he not only misses out on the chance of having his kids live with him, but will have to pay maintenance to their mother, who is able to house them nicely, as she was left with all of their joint assets.
Whatever the law dictates, IMHO, the fairest moral way to work it would be for them to each get 50% of whatever pension pot there is, come retirement time, and in the meantime, sell the £550K house and use the £540K they're left with to buy a house worth approx £380K for her outright and him use the remaining £160K for a sizeable deposit on a smaller house of his own, paying the mortgage from his nice-but-not-megabucks salary, whilst she is mortgage-free.