Okay so I totally think that Peter Capaldi looks like an older Arthur Darvill which works really well, because Rory the Roman and then he plays a Roman in this episode! It just works so well. Even though it's not actually a thing.
Captain Jack has an American accent. They are British because the TARDIS translates the language into what the companion/viewers understand. When translating to English, the default accent is British, for some reason.