You need to keep ship
quality in mind. Against the Bismarck, one single battleship, you deployed the following:
http://www.bismarck-class.dk/bismarck/miscellaneous/britforcedeployed.html
The outcome? The Bismarck sank the Hood, the largest battleship of the world at that time (and a British one). Later on it got its rudder damaged so it could no longer maneuver, fought a big battle against the above forces, and ended up out of ammunition because the British ships were unable to penetrate its armor and sink it. Being unable to fight without ammo, the crew complement of the Bismarck sank the vessel rather than surrendering it.
In that light, you may want to reconsider your 8 vs 80 destroyers considerations.
Though this is verging on off topic a
leetle, I'll bite.
I won't be reconsidering because while yes, a large quantity of British ships were committed to hunting the Bismarck, it was because they could. They could quite frankly spare them. And as for the strategic value of sending out these wonderful new capital ships on attempts to go merchant raiding - well, the results speak for themselves. Bismarck was disabled by an obscolescent Fairey Swordfish torpedo bomber, at which point all she could do was go in circles. It's acknowledged that she would have sunk from the battering she took from the British fleet. Scuttling her was a last and brave act of defiance. As for the other battleships - Graf Spee crippled and scuttled after encountering three British cruisers, Scharnhorst sunk after encountering a British battleship plus escorts, Tirpitz bottled up in Norway until bombers sank her - in fact if you combine that with Bismarck's fate, the real lesson was that battleships were obsolete and aircraft carriers were the true capital ships of the war. Which the Americans demonstrated with consummate skill in the Pacific.
As for destroyers, these would have been the real threat, acting as fast-moving escorts to any invasion fleet trying to cross the Channel. After disastrous losses in Norway, the Kriegsmarine had a total of 8 destroyers available to escort any river barges filled with troops being towed by underpowered tugs - this was at the time of threatened invasion. It would have been worse earlier, just after Norway, when the Kriegsmarine had (in the whole world) three light cruisers and four destroyers - every single capital ship was undergoing repairs after heavy damage. The British Home Fleet (just what the Royal Navy had around the British Isles that is) had 80 destroyers by itself (plus less relevant capital ships). More could have been pulled in from the Med. And German destroyers were not especially advanced compared to British ones. Even if the RAF hadn't prevented the Luftwaffe from establishing air superiority (which they did), the actual plan for Operation Sealion from a naval perspective looks like suicide. Five years later, 80% of the ships that took part in the D-Day invasion were British... the British will go all out to maintain control of the Channel, as crossing it is really the only way of threatening their home ground. Of course, the truth is that the real plan was to terrorise Britain into surrender or making peace - when that didn't work, there wasn't a lot else they could do.
It still seems bizarre that Hitler thought he could intimidate an island without a proper navy.