I was going to suggest comedy at somewhere like Jongleurs in Camden but their horrible website and mention of hen-parties is a little off-putting:
http://bookings.jongleurs.com/jongleurs/venues/2/camden
The best comedy I've seen in London is at the Comedy Store. However, it's not very sociable - you're sat in rows and there's no opportunity for interaction with each other once the show starts. However, there is a comedy club somewhere near Liverpool Street where you sit down at tables and eat while the comedy is happening. It's not the same standard of comedy as the Comedy Store but the standard's generally OK and it's a much more social experience. I can't remember the name but I'll rack my brains and post it up when I remember.
The other suggestion would be a murder mystery evening. It could be naff but I've always thought it would be a bit of a laugh if no one was taking it too seriously.
http://www.squaremeal.co.uk/events/view/1941/Murder_Mystery_-_Dinner_Theatre
