It might have been cool, but we're sure there are some other royal(ish) people who might be suitable for whatever they've got cooking over there. Here're my suggestions:
-- Prince Harry. He's Charles' cute younger son, seems to have a bit of spunk, would probably be game to poke fun at the royal family.
-- Prince. He's musically talented. Would bring his own costumes. He'd be fine with an unpronounceable alien species name.
-- Anne Hathaway. She's pretty. She knows how to play a princess.
-- Sarah Ferguson. She's a duchess. She's got acting experience (or she can make commercials).
-- Fergie. If they can't get the true duchess, the fake one would do. Plus, it would be awesome to kill her off.
-- Oprah. She's the Queen of all Media, and she's already guest starring on 30 Rock.
-- Queen Latifah. Because she's awesome, and can act.
-- T.I. He's the king of the South. He'll do whatever they like.
-- Aretha Franklin - The Queen of Soul. Think of how much fun the wardrobe department could have with her.
-- Will Smith. He's the Fresh Prince. And he has really big ears.