Assuming you'd paid off your mortgage and had adult, financially self-sufficient children.
I'm thinking £500,000 - even if you don't earn any interest on it, that's £25,000 a year for the next 20 years. If you did live longer, then you could sell your home to pay for care fees etc.
Although I am nearly 60 I don't, sadly, have a spare half a million, but a girl can dream!!