I suspect that P&O will be relying on the defence that their ferries are registered and flagged in (Greek) Cyprus.(since 2019) However, whether an employment tribunal will accept that, when employment terms and conditions did not also change at that time, I'm unsure.
Apparently P&O are attempting to buy their way out of legal actions by offering employees whatever amount they would've been due, had they sought a legal remedy. I suspect many will take this, as the waiting time for an employment tribunal case to be heard may be several months or substantially more.
The jobs have not moved. They are still onboard the various ferries. You can't argue that this is a redundancy situation, because the jobs have not disappeared. For example, by automation. If this was a shore based job, where jurisdiction was not in doubt. Then there would be no question that the company has egregiously breached employment legislation designed to prevent just this thing. Prevent or provide for the compensation due.
Unfortunately for the employees, I believe that they've lost their strongest card. That being the ferries on which they worked. If they'd been able to co-ordinate and seized the ferries in question, that might've encouraged DPWorld, the company's owner, to buy off the employees at an enhanced amount.
I see no wish on the part of the RMT to widen this dispute. Nor, do I see the government weighing in to any great effect, because the amount of freight moved by P&O is of strategic importance to UK plc.
I suspect that someone in P&O and/or DPWorld, has taken a look at world events and judged this to be an opportune moment to 'bury bad news'.
DPWorld made a $2.3Bn profit last year. Or similar. P&O's books might not have looked great following the pandemic, but they did take government money where employees were furloughed. They operate a near cartel with competing ferry companies, so it could be argued that the company could've traded it's way out of current losses. But, obviously that wasn't good enough for DPWorld's owners. Profits before people is what this looks like and if it is the case, then as soon as we no longer rely on Middle East oil, the next dictator who wants to invade Kuwait, Saudi, UAE and so on, should not be stopped by British blood and treasure.