In the movie, the Selma star portrays Prince Seretse Khama, who courted controversy in the late 1940s after marrying white British woman Ruth Williams, played by Rosamund Pike.

The romance led to the couple's brief exile from the country formerly known as Bechuanaland, only for Khama to later return to lead the nation's independence movement from the British Empire and become the first prime minister, and later president, of Botswana.

Khama and Williams' son, Ian Khama, has since followed in his late father's political footsteps and he decided to make an unscheduled visit to the set of A United Kingdom during filming in Botswana.

"Rosamund was in the middle of doing a scene and (the President) turned up unannounced in his own helicopter, by himself, landed it, and sat down next to me behind the monitor," Oyelowo recalled at the movie's New York premiere on Monday night (06Feb17). "He turned to me and said, 'I never thought I would see my parents again.'"

The special guest's arrival helped the cast and crew put the story into perspective: "We literally watched him reliving his own history through what we were doing and that was very special for us," Oyelowo tells People magazine. "That was one of the advantages of getting to shoot in Botswana itself."

Seretse served as president from 1966 until his 1980 death from pancreatic cancer at the age of 59.

Ruth, a mother of four, passed away from throat cancer in 2002, aged 78.

In addition to starring in the biopic, Oyelowo also served as a producer, working closely with director Amma Asante to complete his passion project.