His motive to join the terror plot was to justify increasing the military budget against terrorists as the British government was decreasing the military budget sharply and he felt that this act is causing the country to become vulnerable to attacks.
Before London Has Fallen
Lancaster worked in MI5 as he rose the ranks of Counter-Intelligence Chief. Feeling to justify the need to increase military budget when U.K government has been decreasing it sharply, Lancaster secretly acted as mole for the terrorist group led by arms dealer Aamir Barkawi in their plans to attack London.
After the death of British Prime Minister James Wilson, Lancaster was present in the Joint Security Committee meeting to discuss the funeral of Wilson. He listened as U.K. Home Secretary Rose Kenter makes preparations to ensure the funeral is handled without major problems. When they asked Lancaster about the security threats, he stated that they are British nationals who hostile countries which causes for concerned, but he assured that the reports are not actionable, secretly making sure Barkawi's operation in London is secured without British authorities aware of it.
At London Stansted Airport, Lancaster met up with Mike Banning, head of U.S. President Benjamin Asher's security detail, who has arrived in Air Force One with President Asher and others aboard. Lancaster asked Banning if he was early. Banning told him that they are arriving early for security reasons and said fewer know about President's route to St. Paul's Cathedral, the better the safety for him. Lancaster acknowledged.
After the London attacks, Lancaster was present along with Chief Inspector Kevin Hazard, head of Scotland Yard, on the video conference with U.S. Vice President Allan Trumbull and others in the White House situation room. Lancaster told them that Wilson was actually assassinated with poison in his tainted anesthesia and his doctor's body was found dumped a hour before, leading everyone to know that Wilson's funeral was a trap to kill the Western world leaders.
Later, when MI6 agent Jacqueline "Jax" Marshall arrived in Scotland Yard, Lancaster was surprised to see her and asked what she was doing here. She told them they have a mole. To keep up his appearance, he said that he knows and that is it someone with access to the security routes, police uniforms and such.
He was exposed after forgetting to delete his access log. As he was about to evacuate, Lancaster was held at gun point by Jax. He asks her to join his cause and share the 20 million euro payment from Barkawi. However, he cannot convince her and tries his best to reach the gun near his car to shoot her to evade arrest since his high treason charge will result in execution. He was shot to death later by Jax.