Hello, from London to Dover,there are 2 choices:
by train from Victoria Train Station to Dover Priory (I paid 40$ Cnd because I bought my tickets in Montreal before leaving). The second choice and the best: National Express Bus. From Victoria Coach Station to Dover Coach Station: about £24 for 2 adults, less if you are over 60. To get to the ship you can take a cab, it is not very far. We walked from the train...with all our luggage to the ship terminal. Last year we did London to Dover by coach and the driver dropped us at the cruise terminal. Lucky us! We are going again to London this fall (Sidney hotel because it is not far from Victoria Station, less than one Km)
and we will get to the cruiseterminal by coach.
Have a nice trip.