Anyone witnessing a dangerous or potentially unsafe airspace event should report the incident to the Civil Aviation Authority and/or to the police by telephoning 101; if you are directly involved in the incident as an aviation professional you should use the Mandatory Occurrence Reporting (MOR) system on the CAA website.
If, as an aviation professional, a member of the drone community, or a drone pilot, you are concerned that a drone is being flown irresponsibly, intrusively, dangerously or in manner that may have an impact on airspace safety, you can report your concerns via the CAA-sponsored CHIRP confidential reporting programme, which is accessible via the CHIRP website.