Harwich my be a schlep from Devon, but it has the advantage of an a very nice overnight ferry with cabins and depositis you in exactly the right place to start..
You arrive early in the morning and can either start the ride at Hook of Holland, or jump on a train and personally I'd head for the excelent German river/cycle network
Check this map out to see what i mean, there are part of the 10,000 KM surfaced and signposed long distance cycle touring routes they have.
http://www.esterbauer.com/uebersicht_d.html
They put most of the rest of Europe to shame (and makes you realise in the UK we are not even in the game, like the Germans had Autobahns in the early 1930's we did not even start building ours until the late 1960's, and in the 50 years since we still have not caught them up)
Or as somebody above suggested head down the Muse, but you could turn left and then head down the Mosel and join up with the Rhien which would take you back to the start.
Or head up the coast to Hamburg, hang a right and up the Elbe, with a possible deviation to Berlin
The world is your Oyster - I'd start by taking a very close look at the Bikeline Esterbauer books, there are a number in English (including Elbe, Rhien, Danube, and more) get the books and then work out how to get there, remember taking bikes on trains is (mostly) a lot easier in Europe (not TGV or ICE though)
http://www.amazon.co.uk/s/?ie=UTF8&...vptwo=&hvqmt=e&hvdev=c&ref=pd_sl_9mo9axl67l_e
http://www.amazon.co.uk/s/?ie=UTF8&...vptwo=&hvqmt=e&hvdev=c&ref=pd_sl_9mo9axl67l_e
http://www.esterbauer.com/db_liste_int.php?land_id=XE&reihe_id=RB
The books are worth getting (even in German) just for the maps. The text is a bonus if you can read enough to fathom it out