"Strictly speaking I suppose that flight deck crew do not possess a professional qualification as such."
The definition of profession, according to the OED, is a calling or occupation.
Can't speak for other countries but the requirements for a British Airline Transport Pilots Licence do, according to education professionals, (there are two in my family, both well respected), satisfy the requirements for the academic side of a professional qualification. After the academic side there is a requirement for one thousand five hundred hours of relevant flying before the licence can be issued. Licence issue is only the beginning, third mates on a tramp steamer will have a Master's Ticket.
Now comes the experience required and the ability to pass six monthly and command assessment checks, not to mention a command course before said individual can be a captain.
Not sure what the basis of your reasoning is SXB, but I certainly believe, along with most, if not all my pilot colleagues, that I am a qualified professional.