And She Wore....

Alexander McQueen or Sarah Burton, really.
Sarah Burton was always the front runner- but denied that she was busy, living in the Castle, designing this gorgeous gown for months:

 Not only is it gorgeous and a bit reminiscent of "Grace Kelly's Dress", it also has a Modern feel (by Brit standards)... The Deep V, The lace collar, The Sweetheart cut neckline. It's the perfect fit for a Princess.                       

 While I do love the princess' gown, here is a dress that struck me as a surprising pick- but really youthful and chic. I LOVE LOVE LOVE Pippa's Dress (also, by McQueen's Fashion House)

If you look THISCLOSE you can see the detailing on the sleeve and the cowl neck