Charing Cross road is of course a great suggestion. TIme was my manor, Greenwich, had many excellent s/h book shops, but they've been driven out by rising rents.
One good source for old photography books is the Oxfam book shops that are springing up. I've seen some excellent books there. They are scattered around London, and the rest of the UK, but there's a particularly good on one Marylebone High St. It's fairly near Daunts, which is possibly the best new book shop in London.