I went to the skyfall pre-screen in Leicester Sq, mostly it's media companies, their clients, and famous people tbh. I'd be surprised if you can BUY tickets, and if you can, there really can't be many available for the size of the room. This is usually the first showing so that media can write their reviews of the film, after the premiere. As i understand it' you'd struggle to get into a premiere without being A-list or senior in the film world.
I have to say, it was good though. You get some of the cast, the director / head of Sony etc come and talk for a bit ("PLEASE DONT TELL ANYONE THE SPOILERS" etc).
This year it's 26th Oct, but cinemas actually start showing the same night. AND the UK gets to see it a week or so before the US! good times. You could theoretically book a Leicester Sq cinema on opening night?
Alternatively, depending how bold you are - try pinging some media outlets. Find out who is sponsoring/showing/advertising etc, and ask them. You'll find anyone who has any connection (like a big Sony account.. think big radio / tv firms) will have oodles of tickets their staff just pilfer at the last minute as they conveniently don't give them out to their clients.