UK Security Service MI5

The Security Service, popularly known as 'MI5' from their former 'Military Intelligence Section 5' designation is the nearest UK equivalent to the FBI – it is responsible for UK homeland security – tracking foreign spies, terrorists and other threats to the nation within the UK.

Like its sister service the SIS/ MI6 foreign intelligence service, until fairly recently it was a "secret service" whose very existence was not formally acknowledged.
In the modern way, it now has its own website (https://www.mi5.gov.uk/) and recruits directly from interested applicants – obviously they have to pass the 'vetting' process.

The website covers a general introduction to the service, history, recruitment, current threat levels and what to do if you see something suspicious that the service might be interested in…

If you want more information then the website offers try a couple of books:
The Defence of the Realm: The Authorized History of MI5 by Christopher Andrew

and

Inside British Intelligence: 100 Years of MI5 and MI6 by Gordon Thomas

Interesting.