I think you are all perhaps missing the main point of this clause:
BRC Issue 7 4.10.3.4
In addition, where metal detectors are incorporated on conveyors, the test piece shall be passed as close as possible to the centre of the metal detector aperture and wherever possible be carried out by inserting the test piece within a clearly identified sample pack of the food being produced at the time of the test.
To me there is no reason you cant insert test piece inside a pack, you just make one up, label it as so. obviously you would make a new pack each batch so long as practical...and that is assuming that there is no real material variance in the batch from start to end.
for example a ready meal that is frozen could vary a lot but i have worked in situations where a single pack was removed test piece inserted, taped up with BLUE or RED tape and used as a test pack for an entire run. (each run was like 1hr so defrosting of said pack was not an issue.
In any case we dont have to wreck lots of packs, we just need to use a clearly identified SAMPLE pack....also if mad right you can test the MD's ability by placing pack in several orientations to test fail safes etc.