Thanks guys, that's all very helpful. But I have to say, I've often visited London in the past, and I've been to the Sunday swims at the University of London several times. It's certainly a pleasant enough time, if you can accept being in an echoing hall filled with the smell of chlorine (my avatar shows what I'm used to) but my main disappointment was social, because in the past I've hoped to actually talk with people, and it seems that after the swim, everyone packs up and heads out immediately. They say that lately there's been an after-swim trip to a cafe or pub, so perhaps I need to try again. I knew about the Watford and Ewell ("Noah's Ark) swims, but they aren't so easy to get to, and I'm reluctant to drag myself so far away and maybe find that I'm going to spend an evening among other people but with nobody to talk to. And need I say, wanting to talk to people doesn't translate to "I want to meet women"! Apparently some of the Singles Outdoor Club leaders live in the Watford area, and they go to that swim sometimes, but I haven't got any connection to them except encountering them on the walks.
I have been to the Naturist Foundation, with a guy I met via an Internet forum, although we didn't keep up a connection. It was nice; I'd go back if a group wanted to make a trip.
Maybe it's useful background information to say that I grew up in south London until my family moved to the USA when I was 16. Since then I've made numerous trans-Atlantic trips, so I'm reasonably familiar with events and getting around in the UK.