GEORGIE: Kayleigh Caldwell. Prominently visible in the "Narnia" Companion and seen (from the nose up) on the bloopers reel. She would have had the most work, as Georgie's maximum hours would be the shortest.
SKANDAR: "Graham", probably Graham Candy, Don't know what he looked like, but we know he helped Skandar swipe a case of pop.
ANNA: "Ashley". If there was a screenshot of Susan with mice, that was definitely Ashley. Anna wasn't very fond of mice.
WILLIAM: Unknown, so figure that it was Josh Leys.
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Personally, I think that all four doubles should have been worked into the movie, if only for quick cameos. Kayleigh, seen from the front and with a blond wig or something, would not have resembled Georgie at all. I have no reason to believe the other three couldn't have been disguised as effectively. True, the four main players were the stars, but their doubles deserved more publicity than they received.