I suppose wherever we go, it needs to be near good transport links, so that people coming from any direction out of London can easily get there - which probably rules out City?
Given Bella's suggestion of an earlier meet at the Champagne bar in St Pancras, I was thinking of something around there - there is an interesting-looking pub/venue in St Pancras itself, but it looked a bit expensive to hire. Hang on, will look it up again ...