Hire one?? My nephews girlfriend spent £400 and shes not likely to wear a full length ballgown again in a hurry.
Some dry cleaners do special occaison hire.
Theres a shop in Touchwood Solihull that does lots of prom party dresses, all very sparkly. I know its a way to travel but theres a kurt gieger so you could get shoes too. As a last resort obviously because of the distance.
Karen Millens stuff lasts ages, if she stays slim she could still wear it in 10 years time, I'm still wearing stuff from 2000.