I dont know if there is a Forbidden Planet 샵 in 런던 but if so 당신 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. 또는 else 당신 could try ebay 또는 amazon?
Also, 당신 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 또는 cutouts :)