Planning for a Sealion is definitely thinking outside the box. While I am of the opinion that it is not viable, I still appreciate the discussion/debate.
The three issues I can see are:
1.) The UK can rebuild its fleet-and fast. And it will, at the first sign of German transports. Combined with French reinforcements (BB/CA in sz8), UK can drop 2 BBs or BB/2 subs UK1 if it wants (although a UK2 navy build is far more common). This will make keeping any German invasion fleet alive that much harder.
2.) The British Isles start with 15 land units. These units usually consolidate in London on UK1-2. Thus, Germany needs 8+ transports, and even then will probably not be able to take London cleanly in 1 combat. Which brings me to my last point…
3.) UK can drop 30-40 IPCs of land units in London on its turn. If Germany’s fleet survived the mines/UK fleet, if London was invaded and contested, this will end any hope of Germany conquering it on their follow-up turn.
It has been suggested that Germany take its fleet to sz4 and drop in Scotland. If I’m the Allies, I would one-two the German fleet with France/UK, and drop enough units in London to destroy the German army in Scotland. In the meantime, Germany’s efforts in Europe proper would be severely hampered by the land units (16) that are in Scotland and the 48 IPCs worth of Transports (8 trn, right?) that could have been used for, say, 16 Infantry.
It has also been suggested that Sealion will slow down Britain in India by having to spend money for London. As soon as the German navy has been dispatched, Britain can easily spend its full income in India again. US should also be involved in the war effort by now–not a pretty picture for Germany.
On the other hand, German fighters from Belgium can reach London; perhaps this can help offset the cost of transports/troops to invade?