You may wish to try www.centralchauffeur.com they offer transfers from London and the Airports. Otherwise there is a train service from Liverpool Street.
We have been on the Jewel in 2004, starting in Harwich for the Baltic Cruise.
We have done the transfer through RCI. They picked up our luggage separetly at the airport and we took the next available transfer bus to Harwich.
We did not have to take care about our luggage and they delivered it automatically to our cabin.