Hi Galadriel,
I was also going to suggest Felixstowe, but Derek beat me to it. There are three other places that may be helpful. Spurn Point, on the North bank of the Humber & possibly Immingham or Grimsby on the South Bank. Ships have to pass close to these two locations, but not very many large container ships. There is also Gravesend. I haven't personnaly been to these other three points, but judging by some of the photos seen on this and other sites, they can produce some nice results. Most of my photos are taken of ships in Harwich, Felixstowe, Rotterdam and any others that pass the ship I work on.
Many regards
Karl