As of 2019, there are 50+ social swim groups around the UK - community groups of tens to hundreds of people who get together to swim, at their own risk - no payment, no water safety, with everyone taking care of themselves.
Being the member of a group can vastly increase the range and types of swims people go on (and often forges strong 'swim family' friendships).
In recent years some have had to hide, for the volume of new swimmers joining them overwhelms them. Others are still happy to be known to new members. This collection is only of the groups open to being listed. So if you are looking for swim buddies in your area read on!
There is a parallel listing guide on https://www.outdoorswimmingsociety.com/uk-wild-swimming-groups/ which is updated regularly.