As many have said the train is very easy.
Also most cruise lines have shuttle buses that will take you from several points in London direct to pier. Good if you have extra luggage.
Pete, there are taxi's and there are taxi's. What you say may be true for London taxi's that would take you to Dover, but there are 'out of zone' taxi companies that are very reasonable. If you use Dover taxi over London taxi it is probably a fraction of the cost. For example, I like to stay in Windsor. If I take a London taxi from the airport it will cost an arm & a leg, but if I arrange pick up by Windsor taxi it is very reasonable. I'm guessing it's the same for Dover taxi and a good choice especially for couples with bags that would prefer the convenience.