I used to go there when I lived in London (shock horror). It was a very peculiar shop - you picked your book(s), then went to pay at one of a number of of tills where the cashier was enclosed in a cage. It went for floor after floor, with very little apparent logic to the arrangement.
I think it was the ?106 Charing Cross Road that was the location for the book/play about the relationship between C.S.Lewis and a younger American woman (not sure about that, and I haven't seen the film).