I think the GP forgot to put "willingly" in there.

The Brits, after WW2, were exhausted. India had been clamoring for independence for ~50 years. And after the war, you suddenly had a million young men in India, well trained in arms and warfare; these men would have been hard to control with force. So they had no choice. Once India fell (i.e. became independent), the dominoes started falling, and soon the sun had set on the British Empire.