We did this!
Lovely ride. Many people will be fairly familiar with the route up to Harwich. We changed our mind on the way and decided not to do the Mersea ferry option and then got a bit lost in the middle of Colchester after having been drawn in to Sustrans routes (in the past I've bypassed it on the ring roads to the South). Having changed route, I was navigating by Sustrans route signs and I have to admit that their sinage is a lot more reliable than when I have tried to do that in the past - other than in the middle of Colchester!
Ferry was great, es ever (Uta was particularly impressed with that).
Thanks to Olaf for route suggestions through the Netherlands. I'd done some of it before, such as the Kinderdijk windmills (which is stunning) but the waterside route through Rotterdam was surprisingly pleasant and the dyke-top section into Nijmegen was very pretty.
The third day in Germany had a very long and prosaic section from Rheinberg to Krefeld. It had the benefit of being Roman-road-straight and hence fairly quick, but was mostly urban sprawl. That bit could probably do with some tweaking. From Krefeld onwards there is a lovely, quiet path by the Rhine. We didn't find the ferry but the motorway bridge over the rhine is fun, and there is then a very pleasant riverside ride into Dusseldorf (and also a few more good sections afterwards)
The weather was good enough. One very heavy shower in the middle which saw us sheltering under the eaves of a school roof for an hour, but a fair bit of sun too.
Distances were just a teeny bit too long: about 100 miles each day, which meant that we were riding pretty much the whole day. If doing it again, we would add an extra day. Especially if going on to Cologne.
The whole way through the Netherlands and Germany we virtually never had to go on a road - only a few very pleasant country lanes.
Here are our routes:
London - Harwich
Hoek - Nijmegen
Nijmegen - Leverkusen