If the building was built in 16th edition you can not test it to the 18th edition. You can advise to the new editions rules to what applies today.
Look at the last sentence beginning with the word 'You' can etc
Says it all really quote new regs and what applies end off...
So by your logic, you would need to hold a certificate that shows you're competent to test and inspect, for every edition of the wiring regulations you come across.
You would also need to have all the reference books for each edition that you come across.
This is just nonsensical.
We test and inspect to the current edition, we are required to hold a certificate for the current 18th edition.
We report the findings of our test and inspection, and note any non compliance with the current 18th edition, each non compliance being rated as C1, C2, C3 or FI.
We recognize that many installations were designed to a previous edition, of course, but our reason for doing any of this is to assess whether the installation is deemed safe for continued use.
I too am at a loss as to why so many find this a difficult concept to grasp.