The Commissioner of Police of the Metropolis is the head of London's Metropolitan Police Service, classing the holder as a chief police officer. The post is currently held by Cressida Dick, who assumed office on 10 April 2017.
The Commissioner is regarded as the highest ranking police officer in the United Kingdom, despite the fact that his authority is generally confined to the Metropolitan Police Service's area of operation, Greater London. However, unlike other police forces the Metropolitan Police has certain national responsibilities such as leading counter-terrorism policing and the protection of the Royal Family and senior members of Her Majesty's Government. Furthermore, the postholder is directly accountable to the Home Secretary and the public nationally amongst many others (the Metropolitan Police Authority, the Mayor of London, Londoners) whereas smaller police forces are only accountable to residents and their local Police and Crime Commissioner or police authority
More information on the Wikipedia page [1]