Welcome to Flyertalk and welcome to the BA forum Rhodent.

I can see it is a tricky situation, and I'm not sure I am going to be the bearer of good news either. I am assuming here that your flights did in fact operate, if they didn't then you may have a bit more scope. But when applying for the FTV it was made clear on the screen and in this forum that the vouchers would be issued per traveller on the reservation, as opposed to whoever made the reservation. So I think that part was clearly laid out at the time, and I doubt BA will budge on that aspect. You mention about vouchers expiring and turning into cash - there is some consumer pressure to allow this to happen, and some airlines are doing so too, usually where they issued vouchers instead of a refund. But BA did offer refunds for cancelled flights, plus they have now extended the FTVs to 30 April 2023, which gets them off the hook there.

Your best bet is to ask your Ex - via an intermediary if necessary - to make a booking for a flight that is bound to be cancelled. Apart from making this booking in his name, or allow it to be made, he needs not do anything else. Ditto for your own FTV. If that happens then you would get a refund to the original form of payment. As things stand, and given the details provided, I don't think you can look to BA to help, the paths lead to your estranged partner. You probably think that is unfair, but you are actually in a more flexible place than if you held tickets for a future flight and had split up before travel.