Short answer: WWI
Up until WWI Britain's empire was largely unchallenged. Britain had a well trained navy and a very disciplined army. WWI challenged all that. WWI was a land War primarily. Many soldiers were lost. The object was not really to occupy land but to destroy people including civilians. It succeeded in the latter objective.
People at home did not want to keep on loosing loved ones. Better to give other people's land than to give their people. What I've written sounds brutal and selfish. Unfortunately, war is brutal and selfish.