You're correct, GNR4Life. Being from England had nothing to do with my choice of Rachel (although, it's understandable and reasonable that Magneto's daughter could have grown up in England instead of in the US), but more to do with her look and abilities. I think she could reasonably pass as Ian McKellen's daughter and a woman of Jewish decent, and she's a farily competent actress who could pull it off and do so successfully.
And BT18, since they've all but confirmed X-Men 3 is the last film to use this cast of characters (or at the very least, these actors and actresses), Polaris would never make it into a film until a reboot or X-Factor type situation. In any event, there will be no Hugh Jackman or Ian McKellen or Patrick Stewart or anyone, so they'll need a big star in a new branch of a franchise, so why not go for her? I'm all for unknowns, but people are more likely to go see a film featuring an actor or actress they know than a group of people they've never seen before in their lives. Besides...we have no idea how Polaris would be used. She could very well become a major player of the film/films she appears in, using Sinister as a villain and malice as an arc for the character, and having some known talent behind it could never hurt.