Generally, the poor French Navy dooms any attempt at an English invasion. Combine that with more pressing Continental issues, Napoleon had no method of attacking England, which us why he created the Continental System as a means of economic warfare.

The British had two great defenses, a fantastic navy that consistently showed it's superiority over most other navies and the Channel. Since the Channel is the only way Napoleon could attack, if he were to attack he would devote time to crossing with an army which would get attacked by the British home fleet. On top of that, even if Napoleon was to successfully cross he'd be stranded in a hostile country with no supply line. Thus it was deemed that any sort of attack on Britain would be impossible.

Building ships is one thing. Having a navy in the Napoleonic period means that you have to train tens of thousands of men in how to shoot the guns in large ships, thousands in how to safely sail a ship, dozens in how to command a large ship, and a handful of men in how to direct the actions of a fleet. Screw up any of these tasks and you have not a navy, but a gigantic disaster.

The British strategy was to keep a close blockade against French and allied ports preventing the supplies to build ships from getting to the ports, but equally as important, to prevent the French navy from getting any training at sea that might help it become an effective battle force.

In addition to what's been said here, navies are extraordinarily expensive -- by far the most expensive arm of any military branch, including in modern times. Napoleon chose not to spend the money to build enough high-quality ships to match the British navy (if he had the money), nor did he have the patience to take the time necessary to train qualified sailors and officers. His infantry tactics were built around formations of conscript troops that could, with little training, use shock tactics to defeat enemies; his navy similarly spent little time at sea (partly because it was generally blockaded by the British) and little time practicing seamanship. It takes years if not decades to train competent sailors, and Napoleon did not invest in that.

For a good basic read on the cost of navies, John Keegan's "The Price of Admiralty" is worth a read.

One thing to remember is that ships take a long time to build. Timber needs to be cut and aged for a bit, shaped, brought to the shipyard, the all the thousands of small accoutrements, miles of cordage, copper plating, sail cloth, and dozens of gun, even for a small 36 gum frigate.

After 1805, the French were virtually without any ships of the line, and those that remained were stuck in port. It would and did take years to replace the losses. You also need lots of experienced sailors, and more importantly experienced officers to even hope to try to match the RN those would take even longer to replace.

Thank you everyone that responded, it makes a lot more sense now.

The French made at least two attempts to invade Britain, both were quite comically amateurish. There was the 'Battle of Fishguard' http://en.wikipedia.org/wiki/Battle_of_Fishguard & then there was the French backed http://en.wikipedia.org/wiki/Irish_Rebellion_of_1798 which may have worked, but Napoleon (who may have had some involement in the planning of) was in Egypt that time with a large French army so the French couldn't really provide much ground support.

After these two failures, you can sorta see why Napoleon may have been hesitant in launching another invasion. He probably realised he could control things better by land operations, whereas with sea operations, he would have less control.