I don't know about you being allowed back onto the quayside after check in but I do know that friends/relatives are not allowed on board to see you off, this rule is for security reasons. Have a lovely cruise.
No ...Those days are long gone before we needed security.
Visitors are not allowed on board but there is a visitors viewing gallery on top of the Mayflower terminal. I think you need photo ID to enter. From there you can just about shout across to the prom deck (or use a mobile). They can at least hear the band play as you sail and see you quaffing the champagne, although at that time of year it will be dark.
I don't think that Southampton is different to any other port in that once you have been allowed onboard and have your Seapass Card you should be allowed to go ashore again and spend sometime with your folks. The chance, as everyone else has said, of them coming aboard are not high but still worth a phone call to P & O 's offices in Southampton, you could get lucky. ..... Neil.
Having cruised with P&O for many years alas they do not allow visitors on board any more. I would suggest if you are not allowed to use the viewing gallery on departure drive along to Town key its a great position to wave your friends off and dont worry you will be alone.