We have done a similar cruise with P&O who provide a free coach back to your departure airport. Don't think RCI do this so your option is train into London, then up to Yorkshire. There is a train station adjacent to the cruise terminal at Harwich, which goes to Liverpool street and then you have to get to Kings Cross.
We have to travel back to Manchester probably returning from the same cruise on May 17. We have decided to go to Colchester by Taxi then the remainder of the journey by National express. Taxi fare £35 coach fare £ 18 each.