Extremist behaviours in prison are identified and monitored through a robust case management process reviewing Terrorism Act (TACT) and TACT-related prisoners throughout their sentence. Information and statistics relating to extremist behaviours are routinely collected at local, regional and national levels. Related statistics for persons in custody and released from custody are routinely provide as part of Home Office Official Counter Terrorism statistics, published quarterly as statistical bulletins [see link below]. These statistics present details regarding the number of persons in custody for terrorism-related offences in Great Britain, including details of ethnicity, nationality, ideology and religion.
The latest statistics can be found at: https://www.gov.uk/government/collections/counter-terrorism-statistics