Why don't you just tell them to get a cable router ? Saves them confusing a "DSL router" with an "ADSL router".
"DSL routers" as sold at places like PC world are normally labelled as "Cable/DSL routers", i.e. they can connect to both cable modems and ADSL modems.
Try doing a search on the PC world website for "router". You'll see that most of the products are labelled as either "ADSL router" or just "router" or "cable router". There are very few labelled as "DSL router".
I suspect that if your customers are going into PC World and asking for DSL routers, the staff there are asking them what it's for, and as soon as the say cable, they're directing them to the right products.