There's one at a health club in Bicester, referred to by local swimmers as "the M40 ditch" because it is 200m long, narrow, and right next to the motorway. And Queenford Lakes in Berinsfield, but they're only open between 8am and 9am on Sundays in the winter.

I ended up going for a solo swim at Hinksey this morning, figuring that if I've swum there year round already it's probably safe enough even as the temperature drops. And I have bought a cheap cable bike lock to tether my bag to the edge of the jetty or something similar!