When I was living in the UK, my friends experienced the strange pull to duplicate "an American" accent in my presence. The crazy thing was, when they'd do it, they almost always spoke like Valley Girls or Kardashians. "Do I talk like that? Is that how I sound to you?"
They would always say no, and I have to believe in my heart that I do not speak like a Kardashian. When McC was in the UK and kept trying to explain to people that she had just a generic accent, like the one used on American TV. There isn't really a direct equivalent in the UK. RP has a class connotation.
But there is a thing about the West Coast and speaking. And now it's really being studied by Stanford linguists! Here it is on the California Report. And here it is with some additional commentary (featuring the fabulous Californians from Saturday Night Live) from the incomparable Lindy West. Boom. (PS- McC, did you know Lindy West is from Seattle?!?)