Links to get you started on researching UK film/distribution companies.
As you research make a list of subject specific key terms, i.e. "an integrated film company. production slate, etc."
http://www.bbc.co.uk/films/ukmovies/interviews/britplayers/vertigo_films.shtml ( essential videos with interviews/information )
http://www.uk.castingcallpro.com/cview.php?uid=354 (some history)
Working Title Films
UK Production Companies and Studios
http://www.bbc.co.uk/films/ukmovies/relatedlinks/production_companies.shtml ( An extensive list with links to many lesser known UK film companies.)
UK Cinemas and Distributors
Several video interviews with key players on various aspects of the British film industry