Not to validate the idea of the proposal but in May 1940 none of the East coast ports were ready to immobilise themselves. Lowestoft and Yarmouth, for example, could do little to stop troopships arriving at the docks especially if a coup de main had seized the port areas. The defence would be the RAF/RN harrying them across the North Sea and doing the same when they return for more. There are extensive areas of open beaches in addition to supplement the ports. The bigger ports might be harder nuts to crack as they had some garrison artillery. East Anglia is ideal country for movement being flat glacial till and technically a semi desert, were it not for the rivers from the Midlands. Capture these sorts of ports and the landing issue for stores is solved troops can be landed over open beaches if necessary to free up port capacity for stores. There is no Home Guard net to slow and waste the invaders before a counter attack. As usual we come back to the reinforcement issue in a hostile environment. No Norway so no KM losses means that they can delay the RN for a few hours. The Luftwaffe will be hard put to put anything over the far side bar the medium bombers and Me110s given the range. So, while the KM and LW are playing away the French are playing on home ground and probably joining in the North Sea free for all. With the invaders cut off and the KM/LW reeling Germany is invaded and it is all over by Christmas leaving the embarrassing issue of Soviet occupied Poland to be the new elephant in the room. After all, Britain and France went to war to liberate Poland.
OK, a few points.
Just because the east coast ports were unready for destruction as was, say, Dover, they are still unpleasant targets. You do have army and naval personell there, and they are armed. They know the port, you dont.
Now, your only real way of securing a port is by paratroops. Forget any ideas of men in small boats, the ports all had A/S boats which will spot this. So you have to airdrop , walk over and secure the port,while your invasion shipping is off the coast in the North Sea. With at the best Me110 cover, against the RAF fighters, and the FAA torpedo and dive bombers. That's going to end well. This is assuming that none of the RN submarines ambling around the North Sea looking for German ships spot them, and the coastal patrols magically miss them as well.
East Anglia is a lovely place for armoured warfare isnt really true (I used to live there). Its flat, certainly, but there are all these drainage ditches and rivers (its basically a drained swamp, after all).
Not sure how the KM is keeping the RN away for any longer than it takes them to sail there. Its a lot worse for them than Norway, no snowstorms to hide in and ifyou want to protect the invasion you have to stick close to them. While the big ships are sailing south, you get to play with the local destroyer forces, air attack, and submarines (you do realise your planning on landing close to the biggest RN submarine base, yes?) It's likely that the KM isnt just decimated, its completely wiped out.
