I dont think its still in much use though because i've typed the act into the west law database and its not got a 'blue C' next to it which means current, but on the other hand its not got the red square near it either which means it no longer stands.
I don't think the manager of any shop would refuse your change though just for carrier bags, and as far as my shop work experience goes, I've never been taught to use this policy, so I still think its just an old fashioned law that isn't common anymore.