Anodyne
The problem with UK airport security lies higher up the chain of command than the individual gate agent.
Very much so. We have seen central govt take this route for over 30 years. It looks like devolved authority but it is devolving the blame. There are numerous examples. Ditto for your observation:
The purpose of a regulator is to provide consistent proportionate regulation, not to farm out regulation to allow it to shirk its responsibilities.
... and the scanners themselves seem set to different sensitivities - my metal bracelet watch triggers some but not others.
Indeed. On several occasisions I asked the folks at Ronaldsway on the Isle of Man why my boots triggered their scanner but not the ones at Luton or Gatwick. He replied that they were all set to the same level. Obviously not! The answer is that they are very sensitive to folks entering the mainland via the Island, particularly from Northern Ireland. This is a perfectly reasonable approach and upping security is valid. No need to lie.
Why for example when on duty but positioning as a passenger, do I suddenly become a more dangerous person, denied access to my crewroom to get the flight documentation that I will use later to conduct a flight, until the end of the positioning flight when my status magically changes back to that of operating crew, and I am allowed into the places I need to go to do my job?
Someone had blundered.
Theirs not to make reply,
Theirs not to reason why,
Theirs but to do and die.
Alfred, Lord Tennyson
Not much has changed since 1854 ...