There must be somewhere in the SE?
Have you tried the Quays at Mychett?
http://www.trisportuk.org/swimming/OpenWaterSwimming.aspx
I've not swum there myself as i'm not a competent enough swimmer just yet but once i am i will be giving it a go there.
FB..Thats not good news..must be due a water restrictions soon then?
Will the normal lakes be open on BH Monday do you think? Have not used Heron or Datchet at all this year yet.