Bookthrift may be a tiny little shop, but consider this: it is right down the street from a much bigger Waterstones branch, and yet it seems to do just fine. One obvious reason is the location, just doors away from the South Ken tube station. What really sets them apart is the quality of the selection and the good prices. I was able to get three very good books for just under £6 - these will keep me occupied on the tube for the next month or so.
Not only do they have a get 3 for the price of 2 discount, but the books are already marked down. While you might not be able to come in and find a specific book, because of the well-chosen selection, you will almost certainly be able to find something you want and at a very reasonable price.