Wot no replies!??
In August all the locals will be in board shorts & vests only, air temp should be 70- 80, if it's real sunny you might hit 90, water should be 55+, most likely mid-60's, it's fine in the sun.
Both places are a pain in the a$$ to get to using public transport, but doable, neither is real close to central London- JB is about 50 mins from Waterloo station then a 25 minute WALK!! Princes is a 25 min train journey from Waterloo then a 25 minute walk... but depending on which station you start from you might be another hour away! Ask whoever you're staying with which tube station you're closest to- the maps are here:
http://www.tfl.gov.uk/gettingaround/14091.aspx
JB has a great setup, see the kickers here:
http://www.jbski.com/jumps and also has a boat lake.
Princes has TWO cables, separate lakes, the whole site is huge.
http://www.princesclub.com/cable_opening_prices.html
If I was going to RIDE I'd go to Princes. If I was going to have a great time I'd go to JB. There are always good riders at both, but dare I say some locals take themselves a little seriously at Princes, but you never know who is there on any day.
It's a lot of effort to get to eithe but worth it, have fun, enjoy London
Best mail them both to see if you can get slider boards, you will need a helmet and jacket for sure.