She explained the accent thing once during an interview. When she was a little kid her parents moved to London and she was brought up in Crouch End until the age of 17. She learnt to speak English in England, so basically had a strong English accent. When she moved back to the states her English accent faded over time and she picked up her native American. However, every time she comes back to England she falls into an English accent, she can't explain it, it just happens.
its the same for me. I was born in England, moved to Scotland from the age of 5, came back to England at 12 with a strong Scottish accent. I lost it in about a year, but everytime I go back to Scotland my accent changes to a Scottish one almost instantly. And i'm now 36.