Sales of desktop PCs have plummeted over the last year or so. I think desktop PC sales have fallen each quarter for the last year.
So I guess shops are more likely to stock laptops, tablets and other mobile devices.
There are loads of online sites that sell desktop PCs, such as Dell, Novatech, Amazon, eBuyer and loads more.
p.s. Note Windows 8.1 has just come out, replacing Windows 8, so maybe PC World have sold all their stock of W8 machines and waiting for W8.1 machines to come in.