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Plus the question of where the ships are registered. I think P&O's are registered in Cyprus, Bermuda and the Bahamas. Some of the laws of those countries apply to the crew even when they're in British ports. Maybe the crews could have some British labour rights if the govt passed a law that the routes could only be served by British registered ships? But I doubt that's workable - it kind of defeats the object of international shipping.
The British crews on P & O ferries were a bit of an anachronism - the industry raced to the bottom in terms of wages decades ago. Filipino seafarers get about $1000 per month. The British Merchant Navy was once the biggest in the world but it's tiny now. European seafarers still exist but they seem to be in specialist niches like oil industry support vessels and megayachts.
They don't but it's complex. Depends on various things including how much time they spend at sea, on land, and where amongst other things EU based maritime workers do have rights but it's pretty difficult to guess who has which rights as it depends on so many factors.