Not illegal at all tricky, unfortunately. The ships are registered in Jersey, apparently, so not subject to UK Law. What P&O have done is perfectly legal under maritime Law.
Interestingly P&O French staff still have their jobs..... long live Brexit?
It seems Johnson, Farage and Cummings managed to persuade millions of underpaid working class people to vote a certain way so million- and billionaires don't have to pay tax.