I wouldn't ring the RSPCA - they have no legal powers to do anything about it.
You should speak to the shop owners, politely, calmly and armed with facts (if they sell books, find the books on the shelf and show them what's wrong - if not, bring in books) to explain your concerns.
If that doesn't make things change, contact the Environmental Health department of your local council. They're the ones who issue the pet shop licences and book the inspections - they are the only ones with the power to take the PSL away. If the *council* wants to call in the RSPCA that's fine, but you don't feel the need to report them to OXFAM do you?
"My bum has been a bum for a very long time, but that does n't mean I have to listen to what it says."
- Terry Pratchett, Fifth Elephant