Philipa Lepley on Fulham Road does a brilliant sample sale every January. You book a slot and get one of their assistants to help you pick from their dresses and accessories, so still get the one-on-one attention, without their usual price tags!
Gowns are reduced from upwards of £6k to under £2k, and all are available to take away there and then. You can either have them alter them to fit you, or better still, go to Designer Alterations and have them do it.
The only downside is that you need to be a size 8-12 in order to fit the samples, but their dresses are really beautiful
http://www.phillipalepley.com/
Sassi Holfold is a few doors down from the PL shop, and they also do a sample sale, but their selection isn't as big