One would have though it be illegal for anyone to tamper with the evidence. Trial by jury seems a frightening prospect.
Regarding a standard test, I don't think there is one that will ensure rifles won't go over the limit in all circumstances. For a long time, I had observed that heavier pellets produce more power in my PCP. Then I discovered JSB Express (.22), which blew that theory out of the water. Almost any CO2 gun :- test it at 38 degrees C and see what happens. As far as the law is concerned, there is no need for elaboration, the limit is simple and clear.
Personally, I would like to see some margin for error. A common analogy is the driving speed limit; a 10% margin of error is permitted and the fine is proportional to the speed over the limit.
With the right tools, any air weapon can be made to go over 12FPE. I think the anti-tamper at least proves to the jury that the owner did not modify the gun and therefore did not intend it to be over powered. Still guilty, but it is likely to achieve more lenient sentencing. Perhaps someone should produce anti-tamper kits.