The ease of invading Britain is based on other things, primarily the Ai's inability to recognize coastline as a potentially hostile border. Since Britain and Japan have no foreign land borders on their home islands those tend to be very poorly defended. To the AI, Malaya or the Raj seem much more in need of strengthening than do the home isles.
Look at the US deployment when they are neutral. It puts a lot of units on the Mexican border and not nearly that many on its major ports. It simply cannot recognize the main threat properly.
Now if you add a quirky naval system on top of that, Sea Lion becomes even less sporting than shooting fish in a barrel.