When Thomas visits the estate where he hopes to get a job, the owner says he used to entertain royalty in the house before WWI. He mentions the duke and duchess of Connaught, the princesses of Fife, the duke of Argyll and the queen of Spain. All of them were related to King George V somehow: they were, respectively, his uncle and aunt, his nieces, his uncle and his first cousin.
While talking to her grandmother, Edith references Adrienne Bolland flying over the Andes. Adrienne Bolland (1895-1975) was an accomplished French aviator who, on April 1, 1921, became the first woman to fly over the Andes. Her accomplishment went largely unnoticed in her homeland. In 1924 she was created a Knight of the Legion of Honor in belated recognition of her Andes flight.
The boutonnieres worn at the wedding of Mr. Carson and Mrs. Hughes appear to be pinned on, and they should have had the stems placed through the buttonhole on the lapel and secured with a tab or loop on the back of the lapel below the buttonhole. Even in the present day it is considered a faux pas to pin a boutonniere despite so many men doing so. In the 1920s every well-dressed man would have known the correct way to wear a boutonniere.