Battersey Power Station has held a few extreme sports festivals - the Redbull X-Fighters (motorbikes etc), a snowboarding/Skiing thing on artificical slopes/snow etc. It may be easier to get hold that rather than near the London Eye, especailly since the Met police may get a bit ansty about having things that look similar to firearms in such a public place close to/in the Government Secure zone. Horse Guards Parade would be a great location, but seriously unlikely.
One of the major parks may be a good idea. If Hyde park can fit in concerts etc, paintball may be doable.
One of the major parks may be a good idea. If Hyde park can fit in concerts etc, paintball may be doable.