More recently, I went on a "lido crawl" around London. My original plan was to follow Kate Rew's route, i.e. Tooting Bec Lido, then the Serpentine, then Hampstead Heath Mixed Pond. However, I was a bit late starting out so I skipped Tooting Bec; that's my local lido, so I can go there any time, whereas I've only been to the Serpentine once before and never to Hampstead Heath. The water was about 16°C, so it's a bit chilly. However, I've gradually been acclimatising to colder water, e.g. I now keep my shower at 4 notches into the cold zone rather than in the "neutral" position between hot and cold. I've found that the best approach is to get into the water as quickly as possible, rather than trying to ease in gradually. It's cold at first, but I feel warmer once I get moving, and I stay warm as long as I keep moving. If I stand half-in then I just get colder rather than warmer. I did 4 lengths of the Serpentine and 6 lengths at Hampstead Heath; I think both places are about 100m long, so that's 1km altogether.
The Serpentine and Hampstead Heath are both lakes/ponds rather than man-made pools, so the water was quite murky, and visibility was less than a metre. The Serpentine is very shallow on one side, and a bit deeper as you get out towards the marker buoys: I like to stay in the deeper section, so that I won't kick the bottom by mistake, but I can always choose to stand up (e.g. to adjust my goggles). By contrast, the water at Hampstead Heath was much deeper, and I couldn't touch the bottom anywhere. This allowed me to jump in, but it means that you need to be a fairly confident swimmer. Anyway, there was a nice friendly atmosphere there.
At the Serpentine, you pay at a desk (£4) on your way in, and you can also pay 20p for a locker. Unfortunately the lockers are quite small, and the keys don't have any wristbands or safety pins attached, so there's no easy way to carry them with you while swimming. I left mine next to the cash register, but it might be easier just to leave your bags within sight of the water. Hampstead Heath seems to work on the honour system: they have a vending machine selling tickets (£2), but nobody checks them. In fact, the machine was broken when I arrived, so I paid on the way out. There are no storage facilities, so everyone just leaves their bags on the grass near the pond.
The Serpentine Lido is now closed for the winter, and the Hampstead Heath Mixed Pond closes this weekend (19th), so I won't be able to use either of those again until next summer. (It's not really worth becoming a club member when I'd go infrequently.) However, I'll keep going to Tooting Bec, and I'm planning to go to the OSS December Dip at Parliament Hill Lido on 4th December.