There are plenty of actors that live in London and don't completely change their accents. Michael Fassbender, James Mcavoy, and Swedish actress Noomi Rapace to name a few :what:
Fassbender and McAvoy are already English-speaking with their own accent. To get rid of a Scottish accent is pretty hard. And Noomi Rapace does have a slight British accent especially when she's been in London for a longer time. As I said she's younger than the ones you've named and therefore more attuned to take on new accents.
There's also something called a "weak/strong linguistic echo". If you've got a weak linguistic echo you can easily take on other accents. Not everything is about PR. Sometimes life gets in the way.
