There may be two or three titles each by Heinlein and Asimov which you'll normally find on the shelves of your local Waterstone's. Contrast that with Dune, which just about every book shop in the UK carries; and the fifteen or so titles by Philip K Dick which are in print and readily available.
Back in the 1980s, Golden Age US sf authors dominated British bookshops. In those days WH Smith used to be a good bookshop and not a glorified stationery shop, and they'd have shelves of Asimov, Clarke, Heinlein, van Vogt, Vance and the like. Most of those authors' books are hard to find now - Clarke, you can still get, of course. And there's the Vance titles in the Masterworks series. But van Vogt can no longer be found.