I know it's not what you asked, but while I was researching what to do on this same situation, I found International Friends. They have a transfer to Southampton with a stop/tour of Stonehenge on the way. They pick up right at various London and Heathrow hotels with your luggage. Take you to the ship and you get to see Stonhenge. I booked it for my May 15 cruise, it says you get to the ship 1-2pm. It's like a pre-excursion