Alex Kingston (ER, Dr. Who): Sarah Bishop
Valarie Pettiford (The Blacklist, Fosse): Emily Mather
Tanya Moodie (Sherlock): Agatha Wilson
AND
Lindsay Duncan (Sherlock, Dr. Who, Gifted): Ysabeau de Clermont
Read more, here: http://www.radiotimes.com/news/2017-10-26/doctor-whos-alex-kingston-and-lindsay-duncan-cast-in-sky1s-a-discovery-of-witches/
