Every part of London really :D
You have you shop in all 10 acres of Selfridges. Portobello Market for fashion and antiques. Camden Market for a bit of everything. Top Shop because it's damn amazing value for what they sell.
There's plenty of
vintage to be found too. And the
markets are great.
What we really have to do is find out about all the
discount stores as well. That link has the likes of Burberry and Paul Smith, but there are others (such as Aquascutum) that I can't remember for the life of me!