scr1- You are correct.
One thing that we do know is that there is no system in Spain or Italy to issue large numbers of work permits and no incentive to create a system.
Even if they did want to create a work permit system (which they don't) it would take a very long time and then when created it would be very bureaucratic.
There is absolutely no possibility for British crew members based in those countries to stay.
On the other hand Britain will set up an efficient work permit system for those European workers currently in the UK because it was a British decision to leave the EU.