Lord Scallion, being a Lord, was appointed by birth right to a seat in the House of Lords - The upper parliament in the UK. A seat in The Lords was hereditary until a law came in a few years ago to stop it happening, in most cases.
The laws of man: those laws created by man that allow him a seat in the House of Lords.
The laws of nature: the fact that he was born into a family that inherits the right to sit in The Lords.
Put them together and you have a law created by man that allows people who were born into a certain situation (law of nature), to be a Lord.
Decreed simply means stated, or ordered.
However he was so driven to the literary world that he ignored this opportunity/appointment and wrote detective novels instead.