Where to shop in London?

I will (hopefully!) be traveling to London this summer and I want to do some serious shopping. Not for clothes, for gifts and -- dare I say it -- souvenirs. Not TACKY souvenirs, really nice things. Where is the best place to shop all day without having to take a taxi everywhere?
Update: I'm looking for typical 'English' things. Jewelry (not too expensive), books, knick knacks, art, ornaments --- that sort of thing. Food items would be great, if there is no problem shipping it. Thanks!
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