Well, it was lost by fighting a pair of unnecessary wars in which Germany was in the right, and thereby hollowing out Europe of it’s ancient cultural origins. Doing so collapsed the empire. And the people did not immediately replace a global imperial government with a local national one. As such Britain as much as France, drove marxist imperialism just as hard as the soviets.
The problem was not gutting the government OR working with the USA to restore the British empire. One or the other. But in typical British fashion, just as the french lost their civilization at the loss of the monarchy, just as the Spanish civilization died at their loss of empire, and British lost theirs – and became as useless as the french and Spanish.
Between 1830 and 1914 the British broke from germanic civilization and became Diasporics in every possible sense.