We are getting near asking everyone for their input regarding venues and so forth.
The first thing I suggest is the choosing an initial venue, and working things out from there. From the survey, pretty much universally popular was just a general meeting area, that we can meet up sit down and chat, which is public but we can a find a place out of the way.
Generally you want these to be near a main transport hub, but don't let that stop you making a suggestion anyway. Such a venue doesn't have to go together with an activity, even if there are activities going on a these places.
So with that in I mind I have some suggestions:
1. South Bank Centre - this was the choice for a previous meetup, and even if there is stuff on, you can find area out of the way. Last time we found some tables in one corner of the bottom floor.
2. Barbarian Centre - this is similar sort of deal, and the brutalist architecture, like the South Bank (whether or not you like it) has the effect of allowing for meeting point options, out of the way. It tend to be slightly less busy, but it depends. It it probably a little less connected as the south bank, Liverpool street is still a terminal like Waterloo.
3. The BritishLibrary. Sssh! I'm not talking about discussing literature, however in wedged between two major transport hubs, it might be an option, and there is probably good meeting places the we could meet in or around.