I've no idea what sort of contracts will be in place, but I think the port of registry IS an important factor. The port of registry effectively makes the ship a part of the country of registration's territory. Same as an aircraft sitting in a foreign airport.
One of my trips to sea was a tanker, registered in the UK and owned by P&O. We spent the whole 6 months carrying refined petrol and diesel up and down the west coast of Mexico, on contract to PEMEX (Mexican petroleum company).
So despite working for a Mexican company, in Mexican waters, we were paid in the UK and because the ship was registered in UK, we were subject to UK rules and regulations. Our officers were British, the crew Indian (cost cutting even 40+ years ago).
I would imagine the officers and crews on P&O ferries will be similar. They work for a Dubai based company (the name P&O still being used is for marketing and image only, nothing to do with the previous London based company). The ships are registered in Cyprus or Bahamas, so no real connection with the UK other than some of the ports they sail to and from.
A lot is being made about the number of British jobs being lost, but any time I have been on a P&O ferry in the last 20 years, it has been crewed by a league of nations. It will be a very complicated set up when it comes to contracts.