are there any more details about the trip and destination? from what i can see on a map, getting to harwich doesn't require any major water crossing. for trip back to the smoke i'd use a part of dunwich-london route that i rode couple of years ago (manningtree, colchester, tiptree, maldon, e hanningfield, w hanningfield, stock, billericay, ingrave, gt warley, romford etc.). if there is a better route i'd be glad to hear from experienced ones (not via dover - thanks!).
I've worked out a route back here to Dartford Crossing..but it is the Colchester, Maldon, Billericay route but turning south after Billericay rather the going on to Warley