Who was monarch of Great Britain when they were stripped of power?

Charles II retained relative power. George III lost "the colonies" but still retained relative power (despite his temporary deposition). His son, the Prince Regent was the first real monarch to be popularly lampooned, which you can say was the start of the loss of unquestioning respect and support for the monarchy - we'd tried doing without them and it hadn't worked satisfactorily, so now we could have them but behave disrespectfully if we wished, something which is very much noticed today. The British Empire was lost as a result of the Second World War, so you could say the influence and power significantly declined during the reign of George VI and Elizabeth II.