We are going on this cruise and P and O expect you to leave your car at Gatwick and they provide a coach to Southampton to join the ship ...your car will then be waiting for you on your return. Unfortunately we live very close to Southampton so this is not a very good deal for us so we will just make our own way home from Gatwick on our return. But I see you are from Leicester so should be quite straightforward for you to drive to Gatwick park the car and they will bus you to Southampton.
We sailed out of Southampton in January and returned to the UK via Silverjst into Stansted. I rented a car one way from Stansted to Southampton with Hertz, about £75. Very good service with no problems, I would do this again. In your case,you would have to weigh up the cost of this versus the cost of the car park.