London isn't a cruise port, but is served by the ports of Southhampton, Harwich, and Dover. In total, Southhampton, Harwich, and Dover is the twelfth most popular port in the world, as the embarkation point of over three hundred and fifteen sailings every year from Cunard, Carnival, Celebrity and more to Europe, the East Coast, and the Mediterranean.
On average cruises from London are somewhat expensive compared to other ports. See the illustration below for the monthly average cost per day. Cruises scheduled from London during the the peak season of summer are significantly more expensive than the low months of winter and spring.