Branagh is one of my all-time favorites, primarily in his awesome full-length movie "Hamlet", which he directed AND starred in. A project like that would have put most people in the hospital with exhaustion, but not Branagh. He's a powerhouse.
That being said, he just wouldn't have been right for any character in W&I. He's a Shakespearean actor through and through - very correct, very sharp, full of energy.
The only perfect actors for W&I were the ones in it.
... followed by yet another anecdote about his sensitive crimes in a punt with a chap called Norman, who had red hair and a book of poetry stained with the butter drips from crumpets.