Assuming they're retired, there are no considerations about being close to work, so you're looking at just what the local area is like. So if they can afford it, what you're looking at is a nice suburb, probably close to the edge of Greater London. And the nicest of those will be in the north west, south west or south east. That is going to produce a list such as Northwood, Pinner, Ruislip, Eastcote, Richmond, Ham, Wimbledon, Bromley. Do they want to play golf? Then go for Northwood!
I happen to live in a north west outer suburb, that has plenty of old people around, and my widowed mother (now aged 79) was very happy there. She only moved because a retirement village was built in Denham (just over the Greater London boundary) and the idea of being with lots of other people her own age, in a place where an old lady on her own can press the alarm button and someone will come running, appealed to her. And she loves it - her social life has improved no end. It is NOT an old people's home - she has her own home that she paid for, just with the addition of someone always on call if she needs it.
But I digress... if you want somewhere that is just a nice pleasant suburb, you're looking at somewhere around the edge of London.