I'm one of the organisers so I'll do my best to explain. The decision to charge was made by Dmitri and his reply covers the basics. He is the primary organise

I'm one of the organisers so I'll do my best to explain. The decision to charge was made by Dmitri and his reply covers the basics. He is the primary organiser and the guy who established the meet-up back when it was held in someones living room to an audience of 20 people. Today we have over 4,000 members and an average monthly attendance of 500 people. Apparently we are now the largest monthly tech meet-up in Europe and organising such an event whilst maintaining a full-time job on the side takes a huge effort. Despite my best efforts to help, almost all of Dmitri's spare time is dedicated to this event and it's because of his dedication that the event has evolved into the behemoth we have on our hands. Despite having generous sponsors, we only barely cover our costs every month so the event is in the process of being established as a not-for-profit organisation and we are now charging an entry fee so that we can develop a fund that will give Dmitri the resources to help grow the event even further and attract more people like Joel Spolsky and Eben Upton to come and spend time enlightening the London tech community.