Fantasy Centre, N7 8LX
One of London's best science fiction and fantasy bookshops, run by Ted Ball and Erik Arthur. They will buy second-hand books at reasonable rates, and sell rather more second-hand than new material; however, they often have some of the very latest books, as they're a popular place for disposal of review copies.
Although they deal extensively in the higher-priced end of the SF book and magazine market, most of their visible stock is the more interesting paperback SF and fantasy.
They are doing more and more business online, and if you contact them by email you can probably ask if they have your desired books. email@example.com