What we do looks like this!
How it works: Come along for 12:00pm and meet folk down in the Wharfside Cellar Bar (downstairs at the Hornimans at Hays Galleria, near London Bridge). Around 12:20 the pitching starts. Everyone who is going to run a game says what they will run, up to how many players can be accommodated, and what the game is about.
It might be GM led or it might be more of a story game with the expectation that everyone contribute equally. When this is complete everyone says what it is they want to play: and hey, if it's your first time here you you get to choose first in what you play.
At 12:30 your group grabs a table and starts playing! The slot finishes around 4:30pm This process repeats at 5:00pm for the second slot.