In Dorset there are the Jurassic Coast Swimmers in Lyme Regis & Charmouth on Facebook as well as West Bay, DurleySeaSwims, & East Dorset Open Water Swimming club groups. In Devon Seaton Bay Swimmer’s, Beer and Burton Bradstock. Check for groups in your county to practice with.
The uber friendly Outdoor Swimming Society Facebook GROUP (NB look for the GROUP, not page) will help you find others in your area. Plus you can discuss training, chaffing wetsuit remedies, cold acclimatisation, newbie nerves and all other issues.