Agencies 2

In the case of the UK, we have MI5 and MI6(SIS). MI6 is almost identical to our CIA in the sense of being primarily a non-domestic intelligence gathering organization. Obtaining the intelligence through some form of spying or espionage. Most of operations are also Clandestine and Covert.
MI5 is not quite like our FBI. The FBI is likened to a domestic domestic interstate police force. They can arrest people for crimes that fall outside the pervue of the state but also are involved in domestic counter terrorism and counter espionage.
MI5 is more like a domestic MI6 and deals more with counter espionage and counter terrorism. They do not arrest people but work closely with Special Branch, which does have the ability to arrest. They do much more counter terrorism than our FBI (keep in mind, the UK has been dealing domestic terrorism (the IRA) and now other groups for many years.)
MI5 and MI6 both work closely with GCHQ which is similar to our NSA; math,cryptoanalysis, and cyber work. Basically communication interception in anyform and gathering intelligence from this.