FWIW, Mel Gibson is American, as indeed is Nicole Kidman. ( And Olivai Newton-John was born in England - so much for famous Aussies ).
The issue is that people are influenced by what they see on the screen, and many will view historical depictions as factually accurate, ( Braveheart is a case in point, it is historical hogwash ). I think that the casting of this new production of Wolf Hall is more to do with "colour blind" casting and preventing comments to the effect that there are not any prominent non-white roles in the show.