Bloomsbury has, not surprisingly, some second hand bookshops with a good selection of academic books, including philosophy. Two that I'm familiar with are Skoob, at north end of the Brunswick Centre near the Russell Square tube station, and Judd Books on Marchmont Street, only about a block away from Skoob. Also, as much as I hesitate to recommend a huge chain, the flagship Waterstone's on Gower Street has used academic books on the upper floors (and is generally better than other Waterstones have become).

If you're feeling adventurous, you could comb through this list for specialists in your subjects. Since it's not necessarily entirely up to date, check with any shop before making a special trip.