Hey Bridget,
What style are you looking for? Traditional wedding gown (full length, with train, etc),
or shorter style, less formal style? If it's a traditional style you want, and a particular designer, you might do best to look up on the designer's website and check the stocklists and store locations around London that may carry them. Not all shops get to carry all designers. For instance if you know you want to look at Maggie Sottero dresses, on their website store locator for London the shops to visit are:
Chiswick , London W4 4PU
Reflections Bridalwear
261 Chiswick High Road
Phone: 0208 995 9630
London , SE9 1BL
Gillian Roberts
6 St. Mary's Place, Etham High Street
Phone: 0208 8597120
London , N16 6TT
Losner Formals
232 Stamford Hill
Phone: 0208 8007466
London , EC1N 8NX
Pronuptia
95 Hatton Garden
Phone: 0207 419 9013
Searching for the dress is the most fun
Monsoon is having a sale right now, and they have some more casual floor length and short bridal dresses at 50% off right now. Like you said, you will probably have to have them shortened, but they do have some gorgeous dresses.
Teri