These sites are great for establishing the relative success or failure of case study films at the box office. You can later make comparisons major US institutions and their films to judge the state of the UK film industry and the types of films it produces and for whom.
Try the UK Film Council's search engine. They are great for statistics including box office stats., etc.
Another useful site for stats and info. is IMDB. See their left hand columns after finding your institution's films. Search other parts of the site, too.
Wikipedia has got to be a useful port of call for each part of your case study film and its institutions:
Newspaper reviews are often crucial for discovering the reasons why a film succeeded or failed. Newspaper reviews are also a key source for finding out about box office statistics and comparisons with other or previous films in the same genre:
Remember that DVD sales at Amazon (their rankings for the film's respective genres) is another indicator.
Pearl and Dean - business section is really useful for statistics, target audiences, comparisons with films in the same genre, etc.