Have you actually sailed from Southampton on any of the non Carnival cruise lines?
When you do, you might appreciate the major difference in customer service they provide, and understand why those of us who have sailed with, RCI, Celebrity, MSC and NCL would really like P&O to show their passengers the same courteous consideration.
In fact since Princess manage to provide a much slicker embarkation experience, despite having to use the substandard Carnival terminals, I struggle to understand why P&O don't learn from their US sibling.