I honestly don't see what the problem is with hiring a British actor, or an actor of any kind to portray Superman. Story-wise, Superman is NOT American. He's an alien from planet Krypton. And wasn't Superman originally created by a Canadian? I heard that somewhere... Point is, it doesn't matter what kind of icon Superman is. If the actor can look the part, sound the part, and act the part, then he's just fine for the role.
It's the same argument that exploded when Dragonball Evolution was announced. Goku may be a Japanese icon, but his story was based on a Chinese story, and in the context of the story itself, Goku was ALSO an alien from a different planet, NOT Japanese. He doesn't have to be portrayed by an Asian or Japanese actor, and even if he had been, that wouldn't have saved Dragonball Evolution from being a crappy movie.