Wait a second. Why the hell are people complaining about Juggernaut's voice? A couple of reasons why I have a problem with this:
A.) Who the hell goes to watch Juggernaut in a movie so they can hear his spectacular voice acting?
and
B.) Movie lore may be different, but anyone who knows anything about the Juggernaut, is that he's (in the comics) Xavier's step brother. Let's assume, for a moment that, since Xavier has a british accent in the movie (he didn't have one in the comics. he was from New York) at LEAST one of his parents must have had a british accent. You can, from there, assume that it may have been his father, and that Cain may have picked up the accent just the same as Charles did.