The Home Office collects and publishes information annually on the outcome of misconduct proceedings and criminal investigations involving officers, including the number of officers that have been dismissed following proceedings. However, information on a) the number of police officers issued with a police caution and b) the circumstances that led to a case being brought is not collected centrally by the Home Office
The data on the outcomes following misconduct proceedings, which covers cases in the year ending March 2019, is available here: https://www.gov.uk/government/statistics/police-workforce-england-and-wales-30-september-2019.
The College of Policing publish data annually relating to those who have been dismissed from policing and placed on the Police Barred List. There are no categories specifically on extremist material or being members of proscribed organisations but, between April 2018 and March 2019, 17 officers were dismissed under the category of ‘notifiable association’. This data is available here: