ORIGINAL: yammahoper@yahoo.com
One fleet in the channel prevents any French crossing, assuming you use the optional land crossing in the rules (we did most of the time). If the board situation shows a threat of the french attempting to invade, then the Brits need to pick when to move in the naval phase carefully, which should allow them to stop an invasion force.
If your port guns are manned, no enemy fleet can eneter the port to disembark troops, so he will have to do so from the open sea hex or blockade box. Thus, if you engage the enemy fleets and win the battle, no troops will be able to land after they flee to the nearest enemy port.
Is the situation sometimes tense? sure it is. It helps to have the army at home or lots of troops in garrisons, a not so hard to achieve situation after taking a bunch of conquerds. I have had every city in Britain filled with troops, even had to begin placing them in north african cities I had so many. It is a long game, and certainly in the beginning you have to play a bit more cautious than later, but Britain has enough defense to allow for a respectable pffense with both army and fleets. I will note that I was once invaded while my army was in Egypt. I had to disband the army and place new corps in Britain proper to fight the invader and do it with out my cav corp. Was very painful indeed.
yamma
I never understood how anyone could suggest in 1805-1815 that the Channel was a potential land crossing. I have never heard one realistic arguement that validates this option.