Common Law was established by Alfred the Great, who reigned from 871-899AD. He compiled the laws and customs of the nation into the "Liber Judicialis," based on the Ten Commandments and the Golden Rule. Alfred's son, Edward, declared
To all who are charged with the administration of public affairs I give the express command that they show themselves in all things to be just judges precisely as in the Liber Judicialis it is written; nor shall any of them fear to declare the common law freely and courageously.
In contradiction to the Common Law, the Civil Law of Rome prevailed in continental Europe. When William the Conqueror invaded in 1066, he brought with him jurists and clerics steeped in the principles of Roman civil law. Our ancient laws and customs withstood the shock, and remained without any serious amendment. Common Law includes the Charter of Liberties, which makes the Monarch subject to the law, the 1102 Synod of Westminster, which abolished slavery in England, the 1627 Petition of Right, which granted the right to criticise the government without fear of arrest, as well as Magna Carta and the Declaration of Right. Common Law defends property rights and rights to self defence.
Many of our greatest constitutional documents are Common Law documents. These are not Acts of Parliament. Their principles cannot be repealed by Parliament, and when our Monarch swore to uphold the "laws and customs" of the people of the United Kingdom at her Coronation, those "laws and customs" include Common Law.