Wednesday 8 August 2012 2.25pm
There's an excellent swimming pool at the Marriott County Hall
. I think you can get temporary membership. Not cheap but definitely worth it.
Other than that options just outside SE1 include the Oasis Centre, Covent Garden - an outdoor pool. Or the Southwark Council
run Peckham Pulse.
There's a whole thread of ideas here http://www.london-se1.co.uk/forum/read/1/157023