I think it should at least have been tried.
- Starting directly after the French campaign, when Britain was still badly prepared.
- Weapons and tactics of the British and French are analysed, means against them have been prepared.
- Everyone and everything is mobilized for the effort - even pensioneers, if they can still hold a gun (mainly in defensive positions or on simple duties). 2/3 towards Britain, 1/3 towards the Russian and other borders in case others attack.
- Intense u-boat war fare close to the British isles to sink British ships, to sometimes even attack targets ashore, to sometimes land commandos and spys, and so on - even on a suicidal level.
- Masses of activities along the channel to attract and shoot down British planes and maybe even ships or boats.
- An open attempt to dig a channel tunnel - should force the British to react. The real attempt is covered better.
- Masses of aerial assaults against air ports, bridges, road crossings, rails, railway stations, military structures, radar stations / radio antennas, harbors, storage facilities, ground troops, and so on - even on a suicidal level.
- Fake landing preparations to make British ships and planes get in reach of German fighters, torpedo bombers, artillery, torpedo boats, and so on, and to test and analyse the British reaction.
- Masses of torpedo boats, small subs, and so on, trying to sink the one or other ship, trying to shoot down the one or other plane, and so on - even on a suicidal level.
- Masses of fire bombs to occupy the people - on industrial centers, on commercial centers, on woods, on farms, and so on. Also some harmless irritants on the cities, to keep a coordinated defense from happening.
- Masses of decoys to divert enemy forces, produced partly even before the war started.
- A few outdated battle ships, made nearly unsinkable, to divert english forces, to shoot down a few targets (air, sea, and land), to maybe strand and form cannon batteries.
- Masses of cannons on pontoons and boats, to defend the attack - if possible already in the channel, the rest right after landing.
- Masses of floats and pontoons to help tanks of all sizes to cross the channel, produced partly even before the war started.
- Quite a bit of the "real" German fleet to help the effort as much as possible - even on a suicidal level.
- Pontoons and specially made transport subs to secretely transport soldiers to Britain, produced partly even before the war started.
- Lots of paratroopers, supply drops where there are commandos, landings or spys.
- Lots of boats and ships to transport the soldiers - basically everything that swims. Life vests and other aids to increase chances in bullet riddled environments.
- Lots of cement to quickly build concrete fortifications on the beaches from shot tanks, boats, and other available structures.
- Lots of nerve gas made ready in case the English start using gas - but sealed to avoid unauthorized use.
- Additional commandos and a few advance troops land in the evening, usually at remote coasts.
- The main landings happen in the following early morning hours and continue through the day.
- After landing, the troops spread out over all undefended buildings, vehicles, trenches, and other places and things useful as cover, as transportation, or as weapon.
- Civilians and captured soldiers are used as human shields. Children, girls, the ill, and the handicapped are send away to occupy the defenders.
- Close air support helps break down resistance - even if some planes have to land at the beachheads.
- A flexible tactic of expanding where there is little resistance and retreating where there is too much enemy fire helps increase conquered territory quickly and surround enemy positions and troops.
- Subs and torpedo boats try to fend off the British fleet and divert their fire.
- A partly fake resupply effort helps divert British planes and ships further, and helps a little bit if it gets through.
- Deep incursions into British territory help capture supplies, sabotage war efforts, collect information about attacks, laying mines, disrupt communications, isolate troops, and so on.
- A second wave of soldiers in the following night strengthens the successful beachheads and opens a few new ones.
- Additional tactics, weapons, and strategies that just don't come up my mind.
Germany at the time had about 73 million people, it could have easily recruited 7 million of them, using 2.5 million for home defense and 4.5 million for sea lion. Another 1 million could easily have been recruited from occupied territories, Italy, or as mercenaries from third countries. Italy could also have helped with a few boats - if mainly paddle boats. Makes 5.5 million people. I don't know if Britain had enough ammunition to kill half of them, especially considering decoys, dodging, defenses, and so on. I suppose chances are good that such a massive attack, with all the preparations mentioned above, might even have succeeded - though with very heavy losses. A few successful beach heads (surviving several weeks of counter attacks) might be enough to make Britain agree to a conditional peace agreement imo.
Please take it with humour
