I've swapped several books of stamps. I had to stick them all on paper forms - they were the old type you had to lick or wet. It reminded me of collecting Green Shield stamps.

I scanned a couple to see what the 'barcode' said:

JGB S11251018031513573440006603122101               C0D60C8FED03591F01
JGB S11251018031513573450006603122101               EAE016D9138228F401

So each one appears to be unique, presumably to prevent copying or reuse.

Apparantly this us just one of many types of 'Mailmark':
https://www.royalmail.com/sites/default/files/Royal-Mail-Mailmark-barcode-definition-document-September-2015.pdf