Actually the number of actors has expanded greatly in the past decade, and quite a few of them are acting in US TV prime time shows.
Sorry, was referring to the British shows/mini series. I watch mainly PBS which carries the British shows. You seem to have the same people in the period shows. Michael Gambon is in everything!! If I like the series I buy the dvd. Just got "Death Comes to Pemberly" and the actress that plays Elizabeth Darcy is also in "The Bletchley Circle" and "Becoming Jane".