A British actor should NEVER, under any circumstances, be cast as the Man of Steel. Superman is an AMERICAN icon, and he should be kept as such.
And that's not to say that I have anything against the British, not by a long shot. I'm fact, I'm a huge fan of Tony Jay; he was one of the most prolific voice actors of all time, and left behind a lifetime of wonderful voice-acting memories for me; may his soul rest in peace.
But, unless you're just deliberately going for all-out parody, he should NOT be cast as the voice for Uncle Sam.
And I don't care how well he does. Maybe he'll perfectly mimmick an American accent; maybe he'll do Superman so well that it makes Christopher Reeve look like a high school drama student. I don't know, but it's largely irrelevant; it's a matter of principle that only American actors should portray Superman.