This month, the London Wikimeet will be on the third Sunday, January 17th (1pm at the Penderel's Oak).
http://meta.wikimedia.org/wiki/Meetup/London/29
Also, it looks like there will be two London Wiki Wednesdays forthcoming, on Wednesdays 3rd (February and March). Details to follow. The London Wiki Wednesday meetings are more likely to cover other collaborative software such as SharePoint etc but I am sure that will put you off attending, and free pizza and free beer is often (not always) an incentive.
http://www.socialtext.net/wikiwed/index.cgi?london
Wednesday 3rd February looks like a pub meeting, with 3rd March being at a business address (to be confirmed).
Gordon