I saw the movie (it came out March 29th here in the US. I was a huge fan of the book. I love the Tudors, and any kind of historical drama like that.
But I didn't like the movie. For the first half of the movie, the story moved so slow. And then they rushed through years of history in 30 minutes or under. The character development wasn't good. Henry's character, in particular, was sadly underwritten.
I also don't think Portman can do a believable English accent. Her accent kept slipping. Even as an American, I could tell it wasn't a good accent.
But there were plenty of English actors in the film. Most of the cast, writers, director were British. Just because the three leaRAB weren't shouldn't take away from the story. Actors play characters of different nationalities all the time. Portman, Johansson and Bana were probably cast because their name recognition. It seems to have worked, as it had a decent opening (8.3 million on just over 1,000 screens here in the US/Canada.)