Went on IOTS last Saturday and we were on board for 11.45am even though our tickets stated 2pm boarding. We arrived at Southampton at 11.15am and never saw a single queue. I did hear some people say that had been on board since just after 11 although I don't think you can board any earlier than that. I suppose it depends on how quickly the staff & crew can get all the cleaning and stuff done. Staterooms will be available from 1pm so take some hand luggage if you want to use the swimming pools etc as main luggage can take until 7 or 8 to arrive. Have a wonderful time.
You have not stated where your boarding. I am always on board a RCI Ship before 1200hrs, but it's been a good few years since I boarded in the UK so if Southampton is your departure point maybe someone can confirm whether they are up to speed with the rest of the world.
DH was also on Indie last Saturday and he too was there by 11.15 and on board by 11.45. I don't think anyone was on earlier as he had priority boarding (but the wedding party is always allowed on first). I think that is 1:0 to you then and you win!