There are so many cute flower girl dresses around and they all seem to be £50-60 mostly however if you look in the "party" section of most department stores rather than "wedding", there are just as pretty dresses but a fraction of the cost! Here's a fine example from Debenhams in the party section under the brand Blue Zoo. It's only £32 and very cute - will have to check out what it looks like in real life (also comes in pink).
Here's a great offering from BHS -