I echo lots of the recs in this thread already. Here are a few not listed yet :

South Kensington Books - I have a huge sweet spot for this one. It's lovely and extremely close to the museums (V&A, Natural History)

Art words Bookshop in Hackney is very fun and design-y. The Broadway Bookshop is only a few doors down from here.

Near the London bridge, the Riverside Bookshop is a nice stop as well.