I only managed to find the Sense and Scope shops–I think there is or was another one somewhere.
The Sense shop is at 43 Rye Lane and had 13 shelves of books (say 450 volumes in all), leaving aside the separate children’s section and a rotating stand of romance novels (not all of which were in fact romances). There was rather a high concentration of books that looked interesting to me, and in fact I even found two to buy–Bowling Alone (Robert Putnam) and The Red Tent (Anita Diamant). Each of these cost £ 1-50–note that the price is labelled on the cover rather than written inside. This shop also had a sign up saying that they urgently needed donations.
The Scope shop is at 93 Peckham High Street and didn’t seem very book-oriented. There were 3 shelves of books, say 120 volumes in all, and I didn’t see anything that interested me. The common theme in Peckham seemed to be Andrea Camillieri (as opposed to Ian McEwan and Ngozi Adichie Chimamanda in Forest Hill).
UPDATE 13 JULY 2014
This study can now include the ALD Life shop at 45 Peckham High Street.
It had say 30 shelves of books and 1500 volumes overall. The nearest I came to anything I might want was The Dark Room by Rachel Seiffert, but in general the selection looked quite promising. The opening hours are pictured below: