Mr Ghostface said:

There is no in-universe reason for her speaking this way, it's just her performance being, shall we say, variable. It isn't an English accent as such, it's an American slipping into received pronunciation for a few seconds, that's all. As for not sounding British if you are British, well, I'm British and the way she begins her line "Governor Tarkin" sounds a hell of a lot more like RP than any American accent.

I always figured that the RP accent was kind of an aristocratic air she put on, and she slipped back into her regular accent when kicking-butt.

I for one do a much more mid-Atlantic accent at work, and sound very Southern Californian much of the time. It's so automatic I don't even know I do it except when my wife calls me at works and makes fun of it.