Nude swimming at the UCL Bloomsbury
Although Ive not been for a long time the swim at U CL does still happen on a Sunday. I do believe there are times when it doesnt due to pool closure for maintenance, usually during the summer holidays. This link to Naturist London gives you all the info. https://naturist.london/4SundaySwim.php?v=1578915563
Dependant on where you are base there is also a naked swim with a group called Noahs Ark which takes place at Ewell in Surrey (between Epsom and Kingston). Its every Sunday between 4 and 6pm. Whilst it is a much smaller pool being in a gym, it is still possible to swim lengths (they are just shorter so you do more of them!). There is also the added bonus of a sauna and steam room, as well as use of a studio where every week there is a naked exercise class of some kind (yoga, pilates, keep fit) or the occasional life drawing session. There are also occasional aqua aerobics sessions in the pool. The link is https://noahsark.jimdofree.com/