Sooo, after a lot of searching, I found a few pools that MIGHT let people swim with mermaid tails on!!

The first one is Llanidoes, in east Wales (Freedom Leisure). They have mermaid parties, where people can go and use the pools tails
The second one in Bournemouth, on the south coast (Mayim Mermaid). They do have use of a pool.

*Disclaimer...I have not spoken to any of these, check yourself before you go over with a tail*

Hope this helps people!!!