Annoyingly not many UK private fitness type places have decent length pools, and even those that do will be £30/month minimum.

One suggestion I've heard is that some hotels and country clubs offer the use of their gym and for a pretty low monthly fee. Those types of places will probably be quiet. Try looking for local places on TripAdvisor and give them a call.

It might be busy for the holidays though, also most councils are now offering free swimming to 60+ and 16 and under which I'm is part of the problem.