While way behind at 15/1 is Tom Hardy, followed by Zac Efron on 20/1.
Plus Stewart’s Twilight co-star and new Batman Robert Pattinson is on 25/1.
Spencer will be directed by Pablo Larraín, who is best known for 2016 film Jackie, which followed JFK’s First Lady in the aftermath of his assassination.
Speaking with Deadline, the director said: “We all grew up, at least I did in my generation, reading and understanding what a fairy tale is.”