Here's an update on progress. The OOB for the British is now complete. There are 26 Divisions, 4 Brigade Groups and 10 independent Brigades. I've decided to go with battalion sized units for the Brits, so that will be 436 ground units, not including Home Army units. Add 70 naval units and 90 RAF squadrons and that's a lot of units. Divisions will be partly deployed in defensive positions and partly in reserve - created by events when the capture of certain cities triggers them. I've created 2 kinds of British infantry, one with a defensive role for the deployed units and an offensive role for the reserves. Aircraft will mostly start on the map, naval units will mostly be triggered by events. Home Army units will garrison cities and are the only units the Brits can build. Other reinforcements will be created by events.
German ground and air units will be a higher level: Wehrmacht regiments and Luftwaffe Gruppes. My experience is the AI should outnumber you 3-1 to make an equal match, and use of the different organizational levels is consistent with that. As the Germans needed to regroup and reorganize before being ready to invade, research of technologies will drive mobilization of German forces. Garrison and barge units will be the only buildable units.
As I mentioned, in order for Sealion to have any prospect of success, we have to assume weaker British air and naval forces than historically. I'm also adding the option of having the Bismarck and the Graf Zeppelin completed in time for the operation. Not historical, but then neither is the scenario.