Re: Good to see the AAIB keeping up with modernity!
As I understand it (which may well be incorrect!), in pursuance of an investigation the AAIB has considerable powers, takes precedence over a police investigation, and cannot be compelled by a court to release any information it acquires to form part of a civil / criminal proceedings.
AFAIK the intention is that persons involved in an incident are going to be more willing to divulge information to the AAIB (which is then able to produce a better report) knowing that it won't end up in the hands of prosecutors.
So, quite a lot of responsibility. But it's arrangements like this that result in the aviation industry being so incredibly safe compared to, say, walking, driving, etc. That is a good thing.