Well, you can always Ctrl + Spacebar and click on the map to cheat travel

oooh

There are only two routes, none of them direct. One to the east landing in the Netherlands (the route taken by the forces participating in the Walcheren Campaign of 1809) and one to the west, landing near Brest. If there was a more direct or free approach, there would be much easier to invade Britain (or the continent from Britain). This way there's always a bigger chance of finding enemy armies in the Channel, as they have to follow the same route as you, so you better have a strong invasion force capable of dealing with this "blockade".