The list on Government gives you is here:
Let's stop and consider this: Having a PhD does not (apparently) mean you can countersign someone's passport. In fact their examples don't mention doctors but do mention opticians, chiropodists, dentists and registered nurses.
Engineers specify professional qualifications but apparently 'journalist' is good enough on its own, though being a 'scientist' would be a joke profession, I guess? And hey, pub landlords and qualified travel agents are good too (but running a corner shop would be worthless).
Am I wrong in thinking this list seems particularly odd?