The monthly indie pub night is relaxing social with other indie game devs in London. Usually it's quite well attended and all kinds of cool indies show up. Also, silly party games are known to be played.
Here's the website: http://www.londonindies.co.uk/pub-night/
Note: We don't organize this (in fact, I don't think it's organized by anyone in particular), but this meetup-event is just a reminder for the actual meet up.