Berlin to London - Riding in Germany

* B roads (Bundesstraße) are the equivalent of UK A roads more or less - looked after by central government. Some of them do have bike paths, esp near towns, and some of those paths are lovely - offset from the road with trees between. Not all though. Riding on them is like riding on UK roads (though with a slightly higher standard of driving) - it varies from nice to terrifying.

* L roads (Landesstraßen) (aka S roads in some Lands)  are probably the equivalent of UK B roads. They are looked after by the lands/states/counties. Like UK B roads some of them can be pretty busy - but some are lovely and quiet.

* K roads (Kreisstraßen) are C roads and aren't always paved (but often are) - looked after by the district councils (I think thats the UK equivalent).

NB A roads are Autobahns (motorways). You are not allowed to cycle on them.

I would suggest taking a look for the German national cycle routes. In my experience they are built to a very high standard, consisting of quiet roads, on road bike paths, off road bike paths and canal paths. You can definitley traverse them on a tourer. Open Street Map shows these in red if you switch on the cycle layer. Where there isn't a national route in the right place many of the regional cycle routes are pretty good as well - though it probably depends on where you are.

More German roads have bike paths than the equivalent in the UK. OSM should show which.

I haven't done London to Berlin before but I have done London to Paris, Berlin to Copenhagen, Berlin to Warsaw, Berlin to Leipzig et al. I currently live in Berlin so let me know if I can be of any more help.

It might be worth looking at the European long distance cycle network. If you look for Euroroute R1 (or Europaradweg 1) that will get you from Berlin to the coast.

Strava heatmap would help too.

Might be worth looking at the Komoot app - that offers crowd-sourced routes optimised for different cycling styles.