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I am traveling to London next week and I wonder if the post offices are the only places you can buy british stamps?

a few places like supermarkets and shops like wh smiths, if places sell them they have a sticker in the window of the shop saying they sell them

You can even get it in the hotel you're staying in (if its a big one).
Jason

Supermarkets (Asda, Tesco etc), corner shops/newsagents and bigger newsagents like WHSmith will sell stamps. In London there's quite a few post offices to be chosing from too, so you won't be short of places to buy them!