Wikipedia says that The Order of the Redeemer is the oldest and highest decoration awarded by the modern Greek State. There are five classes of the Order and 'Silver' is the lowest of these. The medal of honour is worn on a ribbon on the left chest. The Order is mostly awarded to Greek citizens "who should distinguish themselves henceforth in any branch of public service, in the army and navy, in the diplomatic and judicial corps, in public administration, in the arts, science, agriculture and industry, commerce, or should distinguish themselves in any other social field through outstanding civic virtue, and through illustrious services to the Throne, for the Glory of the Hellenic name and for the welfare of the fatherland". Foreigners, of whom William was obviously one, were admitted either for past services to Greece, or due to their ability "to bring honour to the Order, through their outstanding personal virtues and excellence".
So the big question which, I'm afraid remains a mystery, is why did the King of Greece confer this award to a mostly London based journalist ?