At what point roughly did the practical political power of the UK's Prime Minister p that of the Monarch?

Since the Magna Carta in 1215, Britain's political story has been one of the actual political power very slowly and steadily seeping from the Monarch to the civilian authority, namely the Prime Minister and Parliament. My question is, at about which point did the real political power and ability to shape the policy of the United Kingdom change from primarily the Monarch to primarily the Prime Minister?