Skybrary contains a general introduction to aviation safety occurrence reporting.
The CAA explanation of the Occurrence Reporting System in the UK.
Essentially the UK uses the pan-European system ECCAIRS2
UK Regulation 376/2014 requires the reporting of safety related occurrences involving UK airspace users.
The CAA has created a list of typical occurrences that might be reported, under what it has termed the MORs Code
An MOR is a ‘Mandatory Occurrence Report’.
The CAA also provides the following page to the public for guidance on reporting breaches of aviation law such as low flying or misuse of drones.
There are other resources available to explain the system, for example the guidance provided by the British Model Aircraft Flying Association (BMAA).