Most people who come to London, or live here, go to Oxford and Regents streets for some shopping. I know the place it's packed with people (mostly tourists out and about) but it is a great option if you are looking for the megastores of almost every brand. Topshop, Primark, H&M, they are all massive here. There are huge 'department stores' like John Lewis, Debenhans and, of course, beautiful and unmissable Selfridges. Not to mention the 3/4 floors Hamleys, London's must for parents (and kids). And, all of the expensive more exclusive brands have a spot on either (or both) of these streets.
When I moved here, I found one of my favorites stores right on Regent's Street, only a few steps aways from Oxford Circus tube: And Other Stories.
The reason this place is a favorite: nice designs, different shoes, awesome sunglasses, great cosmetics area, nice lingerie, sales really worth a visit. I think most of my shades are from And Other Stories when I think about it. All of this, at a price that is not overwhelming. In fact, I just paid less than 8 pounds on 2 new sunglasses last week (this is their major sale of course, but when it is not you can find nice ones for 12 pounds!).
They have a great sale going on right now and I have selected a few items that could give you an idea of what the store is about. GO!
& Other Stories: 256—258 Regent Street (W1B 3AF) - Tube Oxford Circus