MI - sorry your dad is the real sticking point about the house, when it's not even him that is sick. What would he do if the carers stop coming? If he were faced with having to do everything for your mum vs moving, what would happen, do you think?
My dad moved to a bungalow with a loft conversion (so there were two bedrooms and a bathroom downstairs and two little ones and a bathroom in the roof space) after my mother died and it was a brilliant move. He left it a bit late and the family house went slightly to ruin around him but he wasn't ready to go before that. He only had a couple of years in the new house, but it was perfect for him, room for all his books, he could potter about, have friends for lunch (he used one of the downstairs bedrooms as a dining room, gotta keep up standards, right?) do a bit of gardening but not be a slave to the mower. It was close to the previous house so all his friends were nearby. He was lucky to find a place that didn't make him feel like he was massively downsizing, but then he was quite a practical man, so that helped.
Dh and I are planning a lock-up-and-leave flat for retirement - safe block, lift, roof terrace for some plant pots, and a service charge to take care of all the crap stuff. At one point we thought this utopia might be overlooking the Thames somewhere, but hahahahahahaha to that now. But we certainly won't be able to run this house in retirement or even past the next 10 years (although in my un-PC moments I have pleasant little fantasies of dd having her wedding reception here - she looks beautiful, the garden looks great and she's marrying a good, kind, responsible man who cooks, cleans up baby puke, is hilariously funny, massively cultured, but who can still put up shelves and unblock drains when required). I do like it round here these days though so even when we move, I think it will be S. London til I die.