The law says that goods purchased must be fit for purpose, and if they are not, you are entitled , not to an exchange, but to a refund, and you don't necessarily have to produce a receipt. However, in this case you may have a problem. Are the labels still attached, or have you removed them? Since you can't prove a date of purchase, could the worn patches possibly have been caused by wear? The only thing you can do is take the shorts back to the shop and explain the problem. If you get no joy, then tell them you will refer the matter to Trading Standards. You'll find their number listed in the phone book - usually attached to your District Council. They're always very helpful. Good luck.