General guidelines are in ICAO Annex 13 ‘Aircraft Accident and Incident Investigation’, which originates from Article 37 of the ICAO convention; the UK is a signatory.
The ‘Manual of Aircraft Accident Investigation’ (ICAO Doc 6920-AN/855/4) provides:-
“… guidance to assist the investigation of accidents involving large modern aircraft and to ensure a logical and balanced presentation.”
Re ‘tc’ comments above – I agree, and add that with an improving understanding of the theories behind human/organizational factors, investigations could engage in greater speculative debate on these issues without deviating from the requirement for factual reporting. In instances where there were indications of HF/organizational issues (nearly all accidents), a discussion could aid safety by highlighting possible issues for others to learn from even if the specifics could not be proven to be associated with the particular accident being reported on.
The Australians, for example, appear to be moving in this direction.