If you're in central London how about Pronovias shop off Oxford Street? Harrods have a bridal section, but a lot of people go there just to try dresses on in Harrods, so they are not very well maintained. There are lots of Bridal shops off Chiltern street just behind Oxford street, there are also wedding planners etc down there. You could visit Browns Bridal which stocks Vera Wang and is opposite Claridges.
I would say choose your dress from somewhere it is easy enough to go back to - so if you live in London, I probably wouldn't get your dress from Yorkshire, you'd be amazed how often you go back to try it on for fittings, to try on your veil, your shoes etc etc! Good luck