This page from GCHQ’s internal GCWiki, dated 24 April 2012 describes procedures for searching through financial data retained by the agency, and cites examples of some of its sources: see the Intercept article Airport Police Demanded An Activist’s Passwords. He Refused. Now He Faces Prison In The UK, 23 September 2017.
This collection of 13 spreadsheets illustrates the relationship between GCHQ and various ministries within the UK government and that agency’s work is by no means restricted to matters of national security, but also extends to domestic issues: see the Intercept article Controversial GCHQ Unit Engaged in Domestic Law Enforcement, Online Propaganda, Psychology Research, 22 June 2015.
This 42-page report from 10 March 2011 provides an overview of GCHQ’s information operations unit and the behavioural science theories that underpin its activities: see the Intercept article Controversial GCHQ Unit is Deeply Engaged in Law Enforcement, 22 June 2015.
These extracts from a GCHQ paper dated 10 March 2011 provide an indication of the agency’s ongoing effects (misinformation) and “online HUMINT” operations: see the Intercept article Britain Used Spy Team to Shape Latin American Public Opinion on Falklands, 2 April 2015.