London is not actually a cruise port, but is served by the ports of Southhampton, Harwich, and Dover. Combined, Southhampton, Harwich, and Dover is the departure port for about seventy sailings per year from Royal Caribbean, Celebrity, Norwegian and more to Europe.
On average cruises from London are rather expensive compared to other ports. Sailings scheduled from London during the peak months of spring and summer are significantly more expensive than the the low season of fall. The graph below shows the monthly average cost per day for cruises from London (Cruises only depart from London between April and November).