Harwich in Essex is one of the quieter UK cruise ports, generally seeing less traffic congestion around embarkation than the larger south-coast terminals, which makes for a more relaxed start to a sailing. The A12 is the usual route from London, busiest at the standard weekday commuter times, so an embarkation day that falls on a weekday morning is where a little extra time in your schedule is most useful.
As with any cruise transfer, luggage tends to be heavier than for a typical airport trip — often a full case per passenger for a week or two away — so confirming your group size and luggage at booking means the right vehicle, whether that is an estate, MPV or 8-seater, is there from the start rather than a saloon proving too small on the day.
Cruise lines typically want passengers checked in and at the terminal well ahead of the sailing time itself, so planning your pickup around that check-in window, rather than the departure time on your ticket, is the more reliable way to arrive with time to spare.