As someone who worked on film premiers for the best part of a decade, I can say they're usually in Leicester Square. Mostly at the big Odeon but sometimes at the smaller one round the corner or the Empire Cinema. Just follow the crowd that will have been there since 9am.
They're sometimes advertised in the Metro paper on the same day.
ha! I doubt it... New hipster-cool fixed gear film
ha! I doubt it... New hipster-cool fixed gear film