I have purchase some beautiful prom/ball gowns from an online site www.dresses.com.
Dresses.com is an American site, based in LA I think, but they mail to the UK. With the great � to $ exchange rate, their dresses are a real bargain, about $300 (�150) for a really well made, gorgeous dress. The last one I bought has the straps and waistband covered in Swarovski crystals.
I received my dressed, well packaged in blue tissue paper, with fabric butterflies scattered over the paper, well within one week. I would highly reccommend them and their site is well detailed.