His name is Kenneth McCallum. MI5 is the British domestic security service, the equivalent of America’s FBI.
MI5, established in 1909 to counter German espionage ahead of World War One, is tasked with protecting British national security. Its main job is currently countering international terrorism though it is also a counter-intelligence agency. It employs about 4,000 people. MI5 said one of McCallum’s top priorities was enabling the agency to seize the fast-moving opportunities provided by technology, including machine learning.
McCallum’s credential include the Skripal case.
After former Russian double agent [Sergey] Skripal and his daughter Yulia were found poisoned with a Soviet-developed nerve agent known as “Novichok” in 2018, McCallum led the agency’s response to the attempted murder, MI5 said.
The British concluded that two agents of the GRU, Russia’s military intelligence agency, were responsible for the attack.
Source: Britain Names New MI5 Chief: The Spy Who Investigated 2018 Novichok Attack – The New York Times