Beckham would be a fabulous Darnley! He even looks like him.
Don't know that Victoria would be right for Lady Bothwell as she is such a scene stealer (see "Spice -the Movie").
I would think Mel B, Mel C, Vic and Emma could do a good job as the four Maries and could do some rock interludes, following the example of the recent Coppola film: "Marie Antoinette". Ex band member "Ginger" (!)would make a good Elizabeth, perhaps in the manner of Miranda Richardson in "Blackadder".