Err... its a small but important point... DSR doesn't give you the right to try goods at home and then return them.
You have a right to cancel. But that assumes you simply changed your mind about the purchase. Once you have opened the box then it is deemed that you have accepted the goods and so waived your right to cancel and return under DSR.
If the goods aren't fit for purpose, or are damaged or in some other way faulty then the Sale of Goods Act (SOGA) gives you certain rights to refunds and repairs. But once again it's not a license to have a try-at-home service. That's not the point of SOGA or DSR.
What a retailer does over and above the requirements of DSR and SOGA is up to them. If they're happy to accept used goods back that that's their choice.
Nor is it an acceptable reason to say that you've returned stuff because "the store is too busy to let you listen to them".
It would be unreasonable to expect a lengthy demonstration if you just drop in on spec on a busy trading day. However, you should certainly be able to book a 30 minute demo slot in advance at any store with a dem room. If the staff are trying to fob you off booking a demo then you have three choices: speak to the manager, speak to head office or take your business to another store.