When I was instructing, I used to regularly bring Trial Lesson flights into London. On Saturday afternoons, and Sunday mornings, London city is closed and its airspace becomes class G. Give a call to Thames Radar just to let them know what you're doing, and there should be no problem. Heading west up the Thames from the Dartford crossing, I used to go as far as Canary Wharf/Millenium Dome. As for gliding clear from 2500' or below, there was London city & Blackheath as my main options. To be honest I was always tempted to do a T&G at London city!!
No idea what the situation is now,post Sept 11, but I regularly hear light singles overhead home at the weekends (Greenwich).