Yeah, London has a fair few. Oddly, though, the
specialists are
getting quite expensive now. Charity shops are the only place for
cheap books, but they almost exclusively deal in trashy fiction,
celebrity books and coffee-table picture books.
For UK readers...
If you can find one, the Oxfam Bookshops are well worth visiting. I've
found quite a few old computer and techncial books in such shops,
including a couple of electrical books over 100 years old.
-tony