Monsoon Bridesmaid Dresses

Where (in England) do good bridesmaid dresses for young adults?

I am looking for bridesmaid dresses for my bridesmaids. They are 10 and 13 and I'm finding it very hard as the childrens dresses tend to be for very little flower girls and the adult ones are too big... I am hoping to go to a high street store for these like BHS or Monsoon... any advice?
I live in the Manchester area.
thank youuuuu!

try this...

http://www.therosedress.com/dresses-gowns-united-kingdom-uk.html