so what do you do with items that have no serial number?? Leads for example. I go to an office full of computers, testing the leads seperatley, they do not have serial numbers to uniquely identify them, and if i give them a unique serial number, how am I going to attach this to the lead?, most often a sticker, which you have already pointed out come off. I think Mr Pat Testing Police Man, you are taking this a little too seriously. 98% of faults are found during the visual inspection and so, when I see a fault and I'm cutting the plug top off and placing a fail sticker on it, I'm not too worried about the serial number.
At the end of the day, nobody takes PAT testing seriously (I appologise if this is your main occupation), especially the customer, and so most electricains don't worry too much about logging each piece of equipment to such detail that we can track it on google earth, especially when some rob dogs are charging 50p an item.
Just realised, when reading your post again, that you comment on duplicate ID numbers in my example I posted, please note the different location description. This was a test on a single company with several departments (ie joinery, building, forestry etc etc) and so, each department started with ID 1.