I'm not sure that witness statements given to the AAIB would be admissible without some kind of verification because they are not given under oath/caution, which is important if someone is being terrible honest and might incriminate themselves or others, so maybe not in accordance with PACE. Although that would lead to an interesting set of questions in court.
The report might be used to identify expert witnesses or others that the Police may wish to interview. The forensic evidence is obviously useful and the I'm aware I have not phrased that correctly so, hopefully, Flying Lawyer can correct me.