There are really many stories about Queen Elizabeth, and you do not really need any particular reason to pay homage to such an important person in world history, but there is a special one that particularly impressed the artist:
In 1998, the Crown Prince and later King of Saudi Arabia visited the Queen on her estate in Balmoral. In Saudi Arabia it was then, and by the way, still not allowed, for women to drive a vehicle. He was extremely surprised when he was sitting on the passenger seat of the Land Rover, and suddenly the Queen swung herself on the driver's seat and slipped him over her property. The fact that the Queen, through her activity as a driver during WW II, was accustomed to driving fast did not make him feel more comfortable, and he asked desperately by his interpreter to reduce the speed.
The English royal house usually resides with the spread of such anecdotes, but this story was itself confirmed by the prince who thought it was "too funny not to repeat."
Such a fine English humour is simply worthy of a note, right?
And for anyone who wonders which prizes are shown on the "stamps": Ist stands for First Class mail, 42 P (Pence) because according to Doug Adams it is the answer to everything, 52 P because Queen Elizabeth rose to the throne in 1952, and 90, because she celebrated her 90^th birthday in 2016.