I feel like kicking myself that I didn't know about any conventions that are held in London all these years. Now that I do know, I have become a regular attendee.

Here's a few in case someone's interested.

STAR TREK LONDON
Star Trek convention held this weekend in UK after a decade.
Location - EXCEL
Cheapest day ticket - £30.
Capt Kirk, Capt Picard, Capt Archer, Capt Janeway, Data, Trip, Tucker, Q, Brannon Braga, Worf, Travis and others attending. Autographs start from £15

MCM EXPO
Held in July and October every year.
It consists of large number of anime and gaming cosplayer attendees ( myself included ), some TV stars turning up and some playable To-be-released games
Location - EXCEL
Day ticket - £10
Usually stars from SciFi channels' TV shows turn up. This year, stars from A town called Eureka and Continuum will be coming.
Autographs start from £15

EUROGAMER EXPO
Held in Sept. every year.
It consists of only a handful of gaming cosplayers and a large selection of To-be-released console games. Lots of goodies available and they sell for stupid amounts on eBay!
Location - Earl's Court
Day Ticket - £12

London Film & Comic Con
Held in July every year.
Location - Olympia
Day Ticket - £6
Stars froma wide variety of shows appear here. 2012 guests were from Stargate, Andromeda, X Files, Star Trek, Charmed, Battlestar Galactica, Heroes, Star Wars and other movies.
Autographs avergae about £15. Gillian Anderson was £45 though!
Autographs start from £15