I think that is what I was considering. My brother bought a house on a similar deal. What happens when they are sold? The backer gets their % of the value.there’s also shared ownership which we have a stake in.
When I got married and we moved house we found my wife had this sort of deal but she didn't know. I had to go see the bank manager to get a loan rather than upping the mortgage. He said you may be surprised but a number of people have requested similar for the same reason.