The Last King of Scotland

In 1970, Nicholas Garrigan graduates from medical school at the University of Edinburgh.[3] With dull prospects at home, he decides to seek adventure abroad by working at a Ugandan missionary clinic run by Dr. David Merrit (Adam Kotz) and his wife, Sarah. Garrigan becomes attracted to Sarah, who enjoys the attention, but refuses to engage in an extramarital affair.
Meanwhile, General Idi Amin overthrows incumbent president Milton Obote in a coup d'Ã©tat. Garrigan sincerely believes Amin will help the country, while Sarah warns him of presidents who have taken over before. Garrigan is called to a minor car accident involving Amin where he treats his hand. During the incident, Garrigan takes a gun and shoots a mortally wounded cow because no one else has the presence of mind to put it out of its misery. Amin is impressed by his quick action and initiative. Amin, fond of Scotland as a symbol of resilience and admiring the Scottish people for their resistance to the English, is delighted to discover Garrigan's nationality and exchanges his military shirt for Garrigan's Scotland shirt. Later, Amin invites Garrigan to become his personal physician and take charge of modernising the country's health care system.
Garrigan soon becomes Amin's trusted confidant and is relied on for much more than medical care, such as matters of state. Although Garrigan is aware of violence around Kampala, he accepts Amin's explanation that cracking down on the opposition will bring lasting peace to the country. Garrigan discovers that the polygamous leader has ostracised the youngest of his three wives, Kay, because she has given birth to an epileptic son, Mackenzie. When treating Mackenzie, Garrigan and Kay form a relationship and sleep with each other, but Kay tells him he must find a way to leave Uganda. Eventually, Garrigan begins to lose faith in Amin as he witnesses the increasing paranoia, murders, and xenophobia in expelling South Asians from the country. Amin replaces Garrigan's British passport with a Ugandan one to prevent him from escaping, which leads Garrigan to frantically seek help from Stone, the local British Foreign Office representative. Garrigan is told the British will help him leave Uganda if he uses his position to assassinate Amin, but Garrigan refuses.
Kay informs Garrigan that she has become pregnant with his child. Aware that Amin will murder her for infidelity if he discovers this, she begs Garrigan for a secret abortion. Delayed by Amin's command that he attend a press conference with Western journalists, Garrigan fails to meet Kay at the appointed time. She concludes she has been abandoned and seeks out a primitive abortion in a nearby village, where she is apprehended by Amin's forces. Garrigan finds her dismembered corpse on an autopsy table and falls retching to his knees, finally confronting the inhumanity of Amin's regime and decides killing him will end it all.
A hijacked aircraft is flown to Entebbe by pro-Palestinian hijackers seeking asylum from agents of international law. Amin, sensing a major publicity opportunity, rushes to the scene, taking Garrigan along. At the airport, one of Amin's bodyguards discovers Garrigan's plot to poison Amin under the ruse of giving him pills for a headache. His treachery revealed, Garrigan is beaten by Amin's henchmen before Amin himself arrives and discloses he is aware of the relationship with Kay. As punishment, Garrigan's chest is pierced with meat hooks before being hanged by his skin.
Amin arranges a plane for the release of non-Israeli passengers, and the torturers leave Garrigan unconscious and bleeding on the floor while they relax in another room. Garrigan's medical colleague, Dr. Junju, takes advantage of the opportunity to rescue him. He urges Garrigan to tell the world the truth about Amin's regime, asserting that the world will believe Garrigan because he is white. Junju gives Garrigan his own jacket, enabling him to mingle unnoticed with the crowd of freed hostages and board the plane. When the torturers discover Garrigan's absence, Junju is killed for his aid in the escape while the plane departs with Garrigan on board. Amin is informed too late to prevent it, while Garrigan tearfully remembers the people of Uganda.
The epilogue shows real footage of Amin, as well as figures such as the 300,000 that died under his regime, and tells of his eventual 2003 death while in exile in Saudi Arabia.

Question: How does Amin  prevent Garrigan from escaping?
Answer: Amin replaces Garrigan's British passport with a Ugandan one.