Yay for American Independence. No, really!

An odd statement for a quasi-neo-imperialist to say, I know, but if the Colonials hadn't (justifiably) got shirty over lack of representation in Parliament, the British Empire would never have become the behemoth that it did.
Thanks to the rebellion/revolution, the British Government changed the way it conducted itself so as to ensure such an incident would never happen again.
Had they not, a revolution similar to that seen in France, 1789, would have very likely occurred on home turf.
The French leadership maintained their "all powerful" façade, and in assisting the American colonies, spent a fortune. This further angered their already irate people, who, in seeing that it could be done, decided to have a little revolution of their own.
So, with the economy booming thanks to renewed transatlantic trade between Fair Blighty and the newly formed United States and with Britain's arch enemy France focused on tearing itself apart, there was nothing to stop the Empire's expansion.

By 1922, it consisted of 458 million people across 13 million square miles.

22% of the Earth's habitable surface.

Thanks, Yanks!