A lot of us have done Calais/Dunkirk to Amsterdam/Rotterdam (Hoek of Holland). For most people it's their first 'overseas' tour
I think you have a couple of routes, depending on time:
You could head south, follow the Canal du Midi via Carcassonne, along the south coast, up the valley of the Rhone, hop right which would take you up the valley of the Mosel as far as Luxembourg, then a straight line to Amsterdam, or follow the Mosel until it joins the Rhine at Koblenz, and then follow the Rhine to Amsterdam,
I've done large sections of that, to do it one go would be amazing and fairly flat
Or head north to the Loire, follow the river as far as Orleans, then join the Seine as far as Paris and then city hop to Amsterdam, the Loire is stunning is you like castles, which will be mostly closed out of season, but this would be at least week or two quicker that the route above
In either case BikeLine books cover large sections of the route with very good maps
http://www.esterbauer.co.uk/international.html
It does not matter what language they are written in, its the maps that count)
For the cycle route between Harwich and London ask on this site as i know they have one, or browse through CycleChart pages as i remember it has been published several times
http://fridaynightridetothecoast.blogspot.co.uk/2015/07/welcome-to-friday-night-ride-to-coast.html