In 2017 I completed the North Sea Cycle Route. Also known as Eurovelo 12, yet also overlays National Cycle Network Route 1. The entire route is approximately 6,000km. Since then, a few ferries have changed, maybe a new tunnel (which cyclists cannot use), and certainely some new national cycleway improvements. An excellent website was discontinued, but plenty of information can be found on this Eurovelo page. It declares it to be 7,250km long, but I am not sure why, as I only omitted Hamburg, due to a violent G20 Summit.
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 you will need to take. Options to take a ferry from Scotland to Bergen ended long ago.
I include a summary map based on Google MyMaps below, which may help with information I gathered at the time, including accommodation I used, and ferries. Click on this link for the native app andfull screen window
Eurovelo 12 – North Sea Cycle Route, as cycled