The interesting thing about this case is that no matter what is in the AAIB report, they can be second guessed by Plod and his pals. The reason they don't investigate accidents in the first place is because they are not trained or equipped to do so. This is outside their area of expertise. At best they can ask a few so called experts, who have a different opinion to the AAIB, for their opinion. At worst, they can... the mind boggles. Remember, all The Filth want to do is nail you against the wall. They are certainly not interested in proving anyone's innocence.
I think that I'd express it slightly differently.
Police objective: find out if anybody has committed a crime, if they have, who, and aim to successfully prosecute them.
AAIB objective: find out if anything from this accident could have been prevented, if so, publish recommendations to try and avoid future similar or related accidents.
Whilst they have similar investigative powers, they have totally different objectives - and so it's inevitable that they will cut across each other.
Realistically - the police must be constantly investigating crimes (or potential crimes) where they don't have in-house expertise, and calling that in from outside.
G