I think it will take a lot of persuading for people to return to big city centres for work.to KK123. As I understand it the children of key workers have been at school all along. There are few enough of them that they can be taught in small safe socially distanced groups. I am concerned for the poor teachers being "sent to the slaughter" in large comprehensives where they move every 40 minutes to another different group of 30 teenagers. I think what you say about kids having to be supervised at home is the main reason why Boris wants them back at school ie the parents can't go back to work while they are supervising kids. I have read that up to 68% of people work from home and though this seems to me a high number, many articles say 38%. They aren't all parents, but nevertheless WFH enables a large proportion of parents to take responsibility for their own children. It is not just the teachers who will suffer if kids go back to school. They will infect their peers and then their parents and then their grandparents. We could easily be in a second wave in a couple of months if schools reopen.
As to remortgaging, obviously it would only be an option for a few people, but at least more of the older teachers who tend to be more vulnerable will be closer to paying off their mortgages. If it was a way out of teaching for only one older,vulnerable teacher it would be worth trying.
Certainly if this goes on indefinitely an idea I've seen and would be happy with is to return to option of retiring at 60 for men and women. It would be good if down to individual choice but I would be up for it and it would free up jobs for younger people.