Nope, she's English. She sounds a bit like Beverley Knight to me at times - who's English, of course.
I'm not entirely sure you can have a regional accent when you're singing. Lots of English and American singers sound pretty similar to me when singing rather than talking.
I've noticed that on occasion. It just felt wierd that she sounded American and the guy sounded English. Since they're in the same band, I was assuming they were from the same place.