Honestly, I think the most common practice is to simply copy the countries from the IMDB. You have something like Harry Potter which is produced by Warner Bros, but is listed as a UK/US film. Dr Zhivago is also an MGM production, but was listed solely as a UK film (although I removed that yesterday since there is no agreement among the sources on what nationality that film is). I think generally it's probably not wise to follow precedents on other articles (in which case we would still ironically end up with a UK/US listing by just copying the IMDB countries), because they all have different standards of editing. I much prefer the alternative solution put forward, in saying what is American about it and what is British; it feels much more encyclopedic to me.