Rupert Ross, 30, and Leon de St. Aubin, 34, must spend at least 30 years behind bars, The Independent reported. They were convicted of ambushing Darcy Austin-Brooks, 20, outside Wandsworth Prison.
Investigators said Ross dressed as a lawyer and mingled with attorneys gathered for meetings with clients. He shot the victim and then jumped on a moped driven by de St. Aubin and left.
Judge Martin Stephens described the crime as a "monstrous attack."
"You both carried out a well prepared and meticulously-planned execution of a man who had become your enemy," he said.
Ross had a long history of drug use going back to his time as a student at Dulwich College in South London, The Guardian reported. Dulwich, one of the elite group known as public schools, was founded in 1619 by the actor Edward Alleyn and numbers comic novelist P.G. Wodehouse, explorer Sir Ernest Shackleton and U.S. mystery writer Raymond Chandler among its alumni.