In 2017 I completed the North Sea Cycle Route. Also known as Eurovelo 12 and encompasses National Cycle Network Route 1. The entire route is approximately 6,000km.
England/Scotland – from London to the tip of Shetland
Norway – from Bergen to the Swedish border
Sweden/Denmark – through Sweden and Denmark
Germany to London – Germany, The Netherlands, Belgium and France and through Kent back to London.
Also I have written about the numerous ferries:
Trail Planner 