It’s rare that a movie can seem to be moving too quickly, but that’s how the beginning of “A United Kingdom” (2017) felt, almost like a montage. However, the story of the biracial love affair that ultimately gave birth to the nation of Botswana would have been at least three hours long if it had followed the typical pace of movies involving Africa, a love story, and lots of people in period garb with British accents. So there’s that. Otherwise, it’s an interesting film, not spectacular, less an artistic achievement than a pleasant trip across a forgotten page of history.