The British Isles fell to Napoleon during the Napoleonic Wars. Which is hilarious since Napoleon was scared shit of invading England due to the English channel and the crazy as fuck tides between England and France. Its the reason why Napoleon went to Egypt and tried to attack its interests there instead of using his navy fleet into attacking England which was his original mission. After the debacle in Egypt(England found out about Napoleon's fleet was parked in Cairo's port, and Napoleon took most of the troops and supplies with him off the shore), Napoleon took the fastest ship back to Paris, bullshitted the whole thing about Egypt and took power from the current powers in Paris.
But yes, Napoleon was fucking scared of the British and Code Geass alternate history is lolcat.