Do they sell frozen rats/mice? If so, do they use the CO2 method, it's the only method the RSPCA allows.

I just smash them against the wall. works every time.

I don't know what a British organization has to do with a US based pet store, but 99% of stores don't kill or freeze their own mice. That's what large scale breeders are for. Also, the RSPCA is completely fine with snap traps, as long as they work 100% properly. Then again, if you have an animal that eats other animals to live, and are worried about an SPCA..I think you should get a dog.