No real answer to your question I'm afraid.
All you state is correct, it is no longer possible to refinance sub prime debt, when you took it out it was indeed the norm to spend a couple of years cleaning up your credit rating and then switch to main stream lender.
If you are in negative equity then you don't even have the option of selling up and starting again.
If you are not then maybe it is an option, however the chances of getting another mortgage then to buy again are probably slim.
Best case scenario possibly to approach your lender with a view to getting temporary leeway with your repayments.
As I say no solid answers available, unprecedented times, no knowing where it will end.