The best book buying deals in Britain are on-line. (A shameless plug for Philabooks|Booksellers). But when I'm replenishing my inventory, which I often do in London because of the ready availability of titles that suit my customers' tastes, I go for remainders.
As is the case in strictly used bookshops in America, discounts in similar London versions are usually no great bargain. (Overhead, etc.) I buy only when I make what I call 'a love connection.' In other words, a book I must have. The used bookshops I like best are in British Museum area.
My favourite remainder spot is South Kensington Bookshop, in South Ken's museum district and French enclave. (It's located in the building complex attached to the South Kensington tube stop). Great titles, many just a year or two old, are be had up to fifty percent off list prices. At times, even greater discounts can be sighted.
The shop seems to be open 'round the clock. At least, whenever I pop in. Service is pleasant and efficient. Highly recommended!
There was another remainder bookshop I used to go to, in Victoria directly across the south side of the station. It used to be a double hit, since I'd also visit the old Politico bookshop, off Victoria Street. (Long gone). Don't remember the name of this Victoria Station shop, and I'm wondering if it's still there. (Memo to self: Check it out on next buying trip).
Meantime, if you follow Philabooks at amazon.com, you'll see some new titles I've just listed. Or, go to PhilabooksBooksellers.blogspot.com. Call 1.215.385.3512 for even speedier service.
-- Richard Carreño