Among the most illustrious guests of Leopold III and Lilian de Réthy were HM Queen Elizabeth II of Great Britain and her husband, Prince Philip, Duke of Edinburgh. In the spring of 1966, on a state visit to Belgium, the Queen made a detour by Argenteuil, where the former King of the Belgians and his wife had been living, in a kind of internal exile, for the past five years. On May 11, traveling by royal train from Liège, the Queen and the Duke of Edinburgh arrived at the little station of Groenendael to be conducted to Argenteuil. Upon reaching the estate, the distinguished visitors were welcomed by a guard of honor consisting of members of a local equestrian club, Les Habits rouges. That afternoon, in the elegant chateau, the two royal couples enjoyed an intimate tea party and pleasant conversation, mostly in English.
Although everyone tactfully avoided mentioning the Second World War, Elizabeth's friendly gesture towards Leopold and Lilian had political significance. During the war, the British Prime Minister, Winston Churchill, had severely, and unfairly, condemned Leopold's surrender to the Nazis in May, 1940. After the war, while Leopold remained in exile in Austria and Switzerland, Churchill had played a major role in preventing his return to Belgium (see, for example, Roger Keyes, Échec au Roi: Léopold III 1940-1951). Publicly, King George VI had been obliged to maintain solidarity with his government and to treat Leopold coldly. Privately, however, he seems to have been much more sympathetic to his Belgian cousin, perhaps partly thanks to the influence of Lord Roger Keyes, the British liaison officer who had bravely defended Leopold from the false accusations of cowardice and treason in 1940. In matters under his control, such as honors and decorations, King George, to his credit, refused to countenance reprisals against his beleaguered fellow sovereign. Queen Elizabeth's visit to Argenteuil, suggesting that she shared her father's sentiments, delighted and touched her hosts.