You won't have time to travel to Cambridge on Friday morning, see the office and get to Harwich before the 10:40 AM boat. You will need to be in Harwich by around 9:30 AM, which means leaving Cambridge around 8 - 8:30 (rush hour). You will really need to be in Cambridge the night before, which means you will only be able to spend one+ day(s) in London. This ain't going to give you much time to see anything, so I would say you need to do some *serious* talking about what you want to see of London in one day - say two, or three major landmarks. Also, bear in mind travel times both around, and out of, London - you need to prepare beforehand to make sure you don't miss trains.

If you arrive on the red-eye at Gatwick, you will probably be able to get to London by about 10 AM. You will be jet lagged, I suspect, and then have to start sight-seeing with a tired little daughter in what will for you be the middle of the night. You will have Wednesday PM (Tower Hill/Tower of London/Tower Bridge???), Wed eve (West end Show, if you can get tickets - then you have to go and see Piccadily and Leicester square - eat at Wong Kei's (Wonkys) Chinese and enjoy wonderful food but the Worst Restaurant Service In The Known Universe), Thursday AM Houses of Parliament/London Eye/Thames Cruise and then around 2 PM you'd need to be heading North for Cambridge. Tight!