The Government takes proscription offences seriously. However, investigations into the activities of proscribed organisations or individuals who may be members or supporters of proscribed organisations are an operational matter for the police and intelligence agencies. It would not be appropriate to publish data on intelligence related matters.
The Government publishes quarterly national statistics on the use of police powers under the Terrorism Act 2000 and subsequent relevant legislation in Great Britain. This includes information on arrests, charges and convictions for proscription offences. The most recent publication up to year ending June 2020, was published on 17 September on GOV.UK: https://www.gov.uk/government/statistics/operation-of-police-powers-under-the-terrorism-act-2000-quarterly-update-to-june-2020.