No one is responding but you could have a look at this website below, the interesting option on P&O is there is a camera in the wedding chapel so friends not there might be able to watch, it is certainly worthwhile asking about.
Hopefully some points from this but hopefully someone who has been involved in a wedding at sea will still come to your aid.
I was married at sea on the Sea Princess in May, I know its not P&O but just to let you know about weddings at sea- it was absolutely wonderful! Everything is done for you, organisation, Reception, etc and the Captain was wonderful, let us take wedding pics on his bridge! And all the other cruisers love a wedding so it was marvellous walking around the ship for pics and having complete strangers coming up and wishing us well. The staff really go out of their way too for weddings and really make it special. I am sure you will have a wonderful time- one tip....just enjoy!