For Napoleon to be actually successful in invading Britain in 1812, you would need the French to get a fleet out of nowhere, which is rather ASB.
The problem is the Landing, not the Invasion in itself. I'm pretty sure Nappy would have enough men and skills to submit Britain if he were to land. However, for the Landing to happen, the French Imperial Navy need to rival if not be stronger than the British Royal Navy.
The French had built a pretty good Navy when they supported the American Revolution. Problem is that, partially because of the French Revolution, most of the good French commanders went in exile or were beheaded and they didn't found very good successors (in the navy at least, contrary to the land army).
Plus, the French Navy suffered two major defeats during the French Revolutionnary Wars and Napoelonic Wars, both at the hands of Admiral Horatio Nelson : the Battle of the Nile (or Naval Battle of Aboukir) in 1796, which doomed Napoleon's Egyptian Campaign, and the Battle of Trafalgar in 1805, which doomed Napoleon's planned invasion of Britain. In both of these battles, the French lost a lot of good ships which were either sunk or captured by the British, leaving the French Navy as a shadow of its previous glory. If you could avoid both Battles, the French Navy could match the British one.
Another solution would be for the Trafalgar Campaign to go differently. Napoleon had quite a good plan at the beginning : lure the British away from their precious Home Islands, then invade. For it to succeed, you would have the French to be more lucky and braver : if Ganteaume's fleet had been able to leave Brest and join with Villeneuve earlier, it could have been trouble for the British. Villeneuve not turning back to Cadix after a naval battle against the British near Brest could also help.
Getting rid of Nelson by having him die early would be another solution, but I don't think his death would be enough. Nelson was the best admiral of his time, but he probably wasn't the only good one the British had.