Section 153 of the Licensing Act 2003 makes it an offence for 'responsible person' to permit the sale of alcohol by a minor unless that specific sale has been approved by that, or another, responsible person.
So where an assistant is under 18, no sale can be made unless the licence holder (or another person, over 18 years of age, to whom relevant responsibility has been given) checks that the customer is over 18 years of age (and also not otherwise barred from the purchase, e.g. because he's already intoxicated).
That's not a 'store policy'. It's simply what the law insists upon. (A store might well have its own additional policy though, such as 'Challenge 25').
http://www.legislation.gov.uk/ukpga/2003/17/section/153
Any of the shop assistants, who were over 18 years old, who allowed a minor to sell alcohol (by passing them a key or card), without themselves checking upon the status of the customer, would be committing a criminal offence. Similarly the manager (who is presumably the licence holder) would be committing a criminal offence is he allowed such practices to go on.
Your granddaughter did not commit a criminal offence but (even if there was no specific training in the matter) she must have been aware that she was breaching the store's policy (even if she didn't realise that such a policy was laid down by law) simply because it should have been obvious to her that there must be a reason why she couldn't sell alcohol directly.