Hi,
We have made the transfer to Harwich both ways, by rail and with RCI transfers. The rail option we chose because we came to Heathrow two days before embarkation and the transfers were not available. The cost for two is not that much different, and you have to manage your bags through Liverpool station (including a long flight of steps), onto the train, and through the station in Harwich. Your wait at the train station, and possible change of trains means you will probably arrive later than the bus transfer. There is another option, via National Express busses. This is a cheaper option, but requires that you get from your arrival terminal to terminal two, and change busses at Victoria bus station in London. Hope you enjoy your cruise out of Harwich as much as we did.
Peter