Posting here for traffic and also with the disclaimer that there's no real relationship breakdown but I've contacted a few solicitors and they all gave me completely different advice.
I earn 3x what my husband does and the deposit for the house was mine. We've always agreed between us that if we ever separate, he'd keep 50% of the equity but I keep the deposit. One lawyer said that was fine as long as we didn't take it court, another said it was doable with post-nup, and a third one said I might as well stay married because it wouldn't end well for me.
It turns out I needed to speak to an insolvency solicitor, but given what my DHs debts are, I actually never had anything to worry about.