Well, the accent his (Australian) actor used is distinctly U.S./Canadian. So's the accents of almost everyone else in the game, minor characters included - except Poison, I think, she sounds English. It's a fair bet that the games's set in an alternate North America. But a few of the references are British, like the Union Jack, and when Vergil called the cameras "CCTV cameras" instead of "security" or "surveillance" cameras.
I think it would have been neat if Kat had been English. Would have diversified her from the other ladies more.