If Napoleon did try to invade Britannia proper, then he would most likely have lost, since British naval superiority would've led to difficulty supplying the invasion force, and the British would not easily let their homeland fall to the French. Furthermore, even if Le Grande Armee succeeded in taking over the British Isles, they would still have to deal with guerrilla warfare, like in Spain. At the very best, it's a very Pyrrhic victory for France. Much more likely, it'll fail, and leave France vulnerable to a more successful invasion by British and their allies.