Yes, possibly you are correct, possibly not...
If it happened in a shop then yes, you have purchased the item and it's yours.
If online they may claim they only pre-authorised your credit card.. this is unlikely to be honest as your credit card company will say it's a sale.
So it probably comes down to this.. the clause in the terms and conditions is illegal.. the same as if a shop put up a poster saying that if they spot pricing errors after you have purchased items they will demand that the items be returned, etc.
You would likely have to check the legislation itself to see if it mentions anything about distance selling.
Have you seen an incorrectly priced item?
Cheers so, Joe