A few points.
You know mortgage is unsustainable, so how best to extract yourself is your query.
...............................
1. If you can rent a comparable house locally for 850 per month ,think about moving.
2. If you can,t rent @850 per month ,stay put for now.
Talk with a PIP (personal insolvency practitioner) the PIP will outline your real options.
If it ends up that Mr SUB -PRIME gives you a poor deal , I suggest just do whatever to protect your family.
If that means they get NIL funds whilst they try to Repo ,it means you have a fund to pay future rent.
I know what I say is (unfairish) to Mr SUB PRIME , but on k43 per year they gave you k 350 !
At worst in your case ,agree to vacate/sell home and NOT be chased for shortfall of circa k200.
In no way do you want that k200 hanging over you.
I have the (notion) that your ongoing 850 to Mr Sub Prime is their best option @ the moment and that is the only reason they have not moved against you.
r