Was Gary Oldman doing an Alex Guinness impression?
If you don't know Alec Guiness played Gary Oldman's character in the original BBC series that this is film is based on, but listening to the way that Oldman speaks in the film I got the impression that he was trying to copy Alec's voice. I find that a slightly bizarre thing to do as he is meant to be playing a character, not do an impression of another actor.
I know that Ewan McGregor did the same thing in the Star Wars prequels and I found it quite distracting in those films as well.