I always felt that Dominic West occasionally wavers in The Wire, though not enough to care. Idris Elba however is incredibly convincing, especially if you hear his day to day voice, which is about as London as it gets.
The guy who plays Marty Crane (John Mahoney) in Frasier; I didn't know he was British until this year! He's from about 20 miles from where my parents live. Conversely, Daphne's 'Manchester' accent sounds nothing like anyone from round here except the students from down south.