Cruise lovers have many opportunities to sail from one of London's cruise ship ports. 
London is an exciting city with lots of history such as the famous Tower of London, artwork, and an English culture that is important to many Americans. London makes a perfect cruise extension since there are frequent flights between London and the United States.
Only small cruise ships can sail up the Thames River to downtown London. Most ships dock on the coast at Southampton, Harwich, or Dover, which are all on the coast a couple of hours from the city.
The London Eye (c) Linda Garrison
One of the most popular is from Tilbury London on the Marco Polo ship run by Cruise and Maritime Voyages.