"I ask the question, why does a gas installer need to be CORGI registered?"
Probably because Gas installers can easily be self employed and for the publics safety its a quick way of establishing competency. This should not and does not apply to Medical Physics who are all employed by a trust and competency can be judged in house.
If the aim is to "strive for the highest possible professional standards" then surely a profiency test would be more appropriate than a simple "how long have you worked in Medical Physics"