GCHQ
Official site of the UK Government Communications Headquarters which is the centre for Her Majesty's Government's Signal Intelligence (SIGINT) activities.
MI5 The Security Service
The Security Service (commonly called MI5) carries out security intelligence work aimed at combating threats to the nation such as espionage and terrorism.
National Criminal Intelligence Service
The National Criminal Intelligence Service (NCIS) is at the forefront of the fight against serious and organised crime in the UK and throughout the world.