AAIB and GA
Resources are limited at the AAIB, however I have consistently been impressed by the depth and completeness of their investigations into fatal GA accidents (fixed and rotor-wing). In all fatal accidents, there will normally be a Coroner's Inquest, at which the AAIB will be called to give evidence. That is always a good opportunity to test the thoroughness of the AAIB report.
As for non fatal accidents which do not result in some form of legal scrutiny.....
From personal working relations with the AAIB (and as a PPL H), I have never detected a commercial/mil bias, or condescending attitude to GA pilots.