So its job roles that are made redundant not people. You can't then take people on in exactly the same roles, that's not legal.
Unfortunately there are any number of potential loopholes to take advantage of here, though. From what I gather P&O are not replacing with like for like roles, they are outsourcing to agency-based employment for 'flexibility' (i.e. not having to have contracts which offer holidays, sick pay etc.) which means the 'employee' roles are legally, technically redundant. It's a glaring hole and should have been closed years ago.
There's also a possibility that those employment laws don't apply anyway as it's a ship - I don't know that for sure, I just know that there are any number of regulations and acts which don't apply to ships and shipping (I work in Health and Safety, and certainly huge swathes of that explicitly exclude ships).
O.