Your present mayor of London is an American.
Alexander Boris de Pfeffel Johnson was born in New York City to British parents because his father went to Yankeeland to study creative writing at Iowa State University. He was given American citizenship as well as British citizenship merely because his birth was registered with both the Yankeeland authorities and the city's British Consulate.
He is of mixed ancestry. Apart from British ancestry, he also has French, Turkish, Russian and German ancestry and calls himself a "one-man melting pot."
Being a Briton who also holds a Yankeeland passport means that he is eligible to run for President of Yankeeland, and there have been those who have been calling for him to run. It would be nice to see a Brit run Yankeeland again.
And the head of the Canadian one was British between 1987 and 1994.And the head of the Brit banking system is a Canadian.