Raoul, I am sure that it is Harwich.
Jeffro, this would be my advice. I would plan on spending the evening in London after the cruise and then departing for the airport the following morning. All three ports are a considerable distance from London's airports and why risk missing your flight? Plan on debarking your ship when you want and then taking a relaxing transfer to your London hotel for some sightseeing, dinner, a great night's sleep and then tackle the flight home the next day? I guarantee this is a more reasonable approach to the end of the cruise.