Princess have a much larger number of smaller colour groups waiting at specific places. They also have a coloured baggage tag counterfoil to show at the gangway. Announcements are made at the various meeting points and those of that colour group disembark together. The idea is that it prevents everyone waiting at the disembarkation doorway, blocking it but then waiting for your group announcement as it is not audible there. I think the difference between P&O and Princess is that P&O do not have a Gatwick early, late Heathrow early, late and people going to Stonehenge, Salisbury on the way to the airport.
Regards John