Gillian Anderson is trying to keep calm after her portrayal of former British prime minister Margaret Thatcher was widely panned by critics. Fans of The Crown slammed the actress, 52, for her “over-the-top” performance, calling it “a bad impersonation” of the political figure and her infamous accent. “The show is a hit around the world, getting raves and winning awards for its actors, but in England, where the series takes place, the reviews have been ruthless,” reveals an insider. “Gillian’s take on Margaret Thatcher was dubbed a ridiculous parody at best.” However, much like the Iron Lady herself, Gillian, who’s married to Peter Morgan, the creator and writer of The Crown, is maintaining a stiff upper lip. “She did meticulous research on Margaret Thatcher, watching hours of footage and working…