Trufflehound
Re-enfranchised
Well, English director, anyway.
One of the four leads was English: Trevor Howard
Screenplay writer was English: Graham Greene
The producer Alexander Korda was half-English.
And the production company was London Film Productions (British).
That last fact is why every source you will see classifies it as a British movie.
Along the same lines, a lot of sources also classify Fargo as a British movie, because it was funded by the UK production company Working Title... So even though that was made in America with American talent and is American as they come, it's actually OURS....