Somebody just asked me where they could find 'more things like this'. It's a good question, and it's harder than it should be. Here's what I know about:
The London Sandpit runs every month, more or less. http://sandpit.hideandseekfest.co.uk/events/. There's a bigger version, called Hide & Seek, once a year. Free.
Iglab does the same thing, in Bristol. http://iglab.urbanantics.net/www/. The bigger version's called Igfest. Free.
The Fun Fed run game sessions every week or so. http://www.thefunfed.com/. They're £10/session, but often the first one is free.
Tin Horse Theatre are up to something. http://www.tinhorsetheatre.co.uk/
That's all I know, in terms of active real-world games within striking distance of London. It's taken me a long time to get that far - I'd love any suggestions.