I saw that film and really don't remember the British being portrayed as Nazis. That being said, I saw it quite a long time ago, didn't think it was that great a film, and so never really watched it again.
I think it might have portrayed a certain character, who was in a leadership position, as a bad guy but not everyone. And as we know, good people do bad things in war, especially if led by unscrupulous or indifferent leadership.
That being said, I don't recall the movie claiming that it was telling a factual story. It was a "rah rah" film about the American Revolution. And, heaven knows, there have been films made about "questionable acts" committed by Americans, especially post-Vietnam so, it's not like Hollywood always paints Americans as the good guys.