I have telephoned P&O and arranged for a bottle of champagne to be in the cabin of some friends when they embarked. It doesn't have to be a package.
You can ring P&O in Southampton and arrange a celebration package but I don't know about a single bottle of wine. We have had a celebration package in the past and found it to be very good value for what you get. Alternativly log onto the P&O official website there is information about special occasion packages etc. Have a great cruise.