Just like the FBI and the CIA, It appears that “MI6” does not want to reveal their “sources and methods” which include criminal acts, sedition, subversion of American laws and the Constitution, while lying and obfuscating their activity from the people of the world.
https://www.zerohedge.com/news/2018-11-23/mi6-scrambling-stop-trump-releasing-classified-docs-russia-probe