A British couple was found guilty Tuesday of keeping a Nigerian immigrant enslaved for more than two decades, forcing him to work for no pay and threatening him with deportation if he tried to escape, prosecutors said. Dr. Emmanuel Edet, 61, and Antan Edet, 58, held their victim captive from the time he was brought to Britain when he was 14 years old, the Crown Prosecution Service said. The Edets were convicted at Harrow Crown Court in northwest London on charges of child cruelty, slavery and assisting in illegal immigration. They told their captive — whom the BBC identified as Ofonime Sunday Inuk, now 40 — when they brought him from Nigeria in 1989 that they would pay him and give him an education, prosecutors said. Instead, he was forced to cook, clean, garden and care for the couple’s children without pay for up to 17 hours a day, they said.