Tom, have done some digging for you. I have it on good authority that the first two are probably suitable:
Lee Valley athletics training centre (North London) -
http://www.visitleevalley.org.uk/en/...letics-centre/ Was the UK athletics training base up to London 2012 Olympics
Crystal Palace weight lifting gym / power lifting club. Old school but loads of platforms. Run by an ex-GB weightlifting coach (Keith Morgan) who helped run the WL comp at London 2012. Can give you contact details if you want.
Other options that may or may not work:
Wasps RFC - pro rugby club with enough kit. Might be open to a seminar during the off season, particularly if the S&C guys / players can attend.
Bisham Abbey (where the England rugby team train during the international season - c.40 guys in camp for weeks at a time, they are likely to have lots of kit)
CrossFit South London (
http://cfsouthlondon.com/crossfit/)
St Mary's University (
http://www.smuc.ac.uk/sport/gym-membership.htm). New facility, has 10 Olympic platforms.
Surrey Sports Park (
http://www.surreysportspark.co.uk/sp...ioning%20Room/). Harlequins (pro-rugby side) train here, but not sure how much kit they've got