I worked in Mortgage debt recovery for a major Bank, so may be able to offer some insights.
I would wager that the reason no one has sorted this, is staff failings. Me and my colleagues were always appraised by how many customers we dealt with in a day. Many staff would call customers who were in arrears, that customer would make an excuse, so the staff member would move on to the next call within minutes. Their stats were great because they were speaking to lots of customers, BUT it was ineffective, as no one had really helped to resolve the issue, ie. why is the customer in arrears and what can the Bank do to help? Myself, I would always spend time with the customer, maybe rearrange their terms (make the term longer so the payments became smaller), so that there was a resolution, but of course that took time, so my stats didn't look as good! Madness! A really stupid way to assess performance, as it drove the wrong behaviours.
Anyway, if the Bank haven't started repossession yet that's good. As I see it, you need to sell the house, and it's up to you whether you do the repairs to get £550k, or sell it, as it is, for £450k.
I would be asking myself, how much would the repairs cost? Can your Dad realistically live through such repairs?
It's important that you sell it before it gets repossessed, because repossessed properties often get sold for way under value. The bank will only care that they get their £108k back. If they got an offer (for say) £350k, they will just let it go for that amount, because their debt will be repaid.
A pp said that if the Bank repossessed the house, that your Dad gets nothing. That's not true. He gets back whatever is left over. So, if the bank sold it for £350k, your Dad would get £242k back (minus any fees of course). But your dad would have lost all control over what the house was sold for - much better to sell it now, and keep control over what you sell it for.
Personally, if I were in your shoes, I'd look now to market the house at £450k, pay off the mortgage, and Dad is left with £342k. Then look to get him moved into a retirement village. My own Dad is in an Extra Care village, and it's fantastic. Everyone has their own independence, but there's loads of stuff on site, like a carvery, a bar, a theatre, a gym, gardens etc.