I use lake 32 in South Cerney. I pay £75 per annum for 365-day access to the lake. This enables me to swim every day until 10am and Tuesday evenings from 6pm. In the OW season, they add Thursday evenings. I can sign guests in for £5 a session.
Lake 12 has OW sessions but last I I looked they were only operating for maybe 6 weeks of the year and only on a Saturday and only for 90 mins - 2 hours at a time.
Fairford is local to me, but you would have to come up with a fairly decent offer to get me away from Lake 32.
PS Lake 32 also gives me free entry to the beach 