If you can't find it in the UK there are sites that will ship from america for you. They give you an american address, you order it to be delivered there and they forward it on for a fee. Bit of hassle but I use it often for things you can't buy in UK.
If you have no luck finding it in the UK there's a webshop here in the Netherlands that sells it in S, M, and L. XL is sold out. I don't think they ship internationally but I can forward it for you if you'd like. Here's the link for reference it's all in Dutch though.
Ah - I'm 5'8" so I really think it won't be to the knee on me :-( Maybe it's just not meant to be ! Have a friend going to states soon - so will ask her to buy it if she sees it - as could be a good summer dress anyway ! Just not for this particular do !