“...and lose all it’s guts from just the past 30 years?”
The process began in 1939, when the Great Empires of England and France were eventually demolished by Germany, Italy and Japan, and then surpassed by the United States and Soviet Union in terms of raw military and economic power, and were shown up for the antiquated, ineffectual and rotten institutions that they really were. Particularly since neither country put much money (or effort) into the defense of the Empires that had made them so “great” to begin with.
Yes, in 1900, one-fifth of the world’s population were subjects of the British Empire.
How did a relatively small number of British control so many Indians and Africans? Because, the British had developed an air of invincibility around them.
When the Japanese defeated them in places such as Singapore and Burma, even the British knew that the gig was up.