I understood from the OP that both London properties are currently rented, with DP’s covering his mortgage and OPs providing her income stream as it is not mortgaged. DP doesn’t want them to sell either as they constitute their pensions.
Seeing as it was gifted fully purchased, then I cannot agree with your following take:
”she has been effectively paying his mortgage by providing him with free housing. He will eventually own that asset, by himself, outright. He thinks of it as his pension.
Currently the OP has no such pension, despite having greater personal assets.”
In reality, OPs parents have fully paid the mortgage and so when there is no mortgage payments to make, there is no one in that situation paying any costs except the living/running costs, (covered by DP).
They have both enjoyed free housing so it’s not altruistic on the part of the OP to not charge her DP (& father of children) something that costs her nothing, in particular when the trade off is that all running costs are covered.
That’s not a bad thing imo, it’s teamwork. I can imagine it worked well and felt generally fair both sides.
What isn’t teamwork or fair is the acquisition set-up and unfair income-funnelling re the new house. It is fundamentally unfair. (Notwithstanding obviously a cast-iron ring fence around the inherited deposit).
As there’ll be a mortgage on the new property which will be based on the DP salary/income, if they did split up then this will mean that he is unable to borrow as much for any other property as he’ll have the existing lending. So it’ll either need to be sold, remortgaged under OP (who will then need to find an income for whatever amount is needed to satisfy the lender) or he’ll be trapped named as sole borrower on a mortgage for a property solely in op name.
It’s not just op who should protect herself in this precarious scenario, especially when DP came in with less to start with and seeing as his income isn’t appropriate “allowed” to be used to pay off a mortgage (that he has been assessed by the lender for, seeing as op income would not be counted for this purpose).