I dont know if there is a Forbidden Planet shop in London but if so you should try there, because we have one (in Belfast) and they have a lot of that sort of thing, like twilight and new moon merchendise, and a lot of posters. Or else you could try ebay or amazon?
Also, you could try putting your name down in a local cinema for a poster whenever the promotional period is over, I know my friend did that for things like x-men and got posters or cutouts :)