As we have discussed already, proving you have Covid is, ironically, very difficult.
But there are other aspects to consider.
Say you saw your sister/brother/friend a week before your cruise. They then test positive for Covid.
Theoretically you should declare this to P&O, and you and your travelling companions will not be allowed to cruise.
P&O say that you should claim from your insurance company - for simplicity lets say Holiday Extras, because, after all, they are recommended by P&O.
Unfortunately, HE say that as the UK government do not regard close contact to a Covid sufferer as a reason to deny travel, they will not pay out. So, you lose everything.
No one in their right mind is going to admit to knowing anyone with covid, or, indeed having covid themselves, because to do so is to basically lose the entire cist of their holiday.