The AAIB are great at what they do but have no real powers, they are just a neutral voice.
The CAA have the powers that can implement the recommendations of the AAIB, if they see fit.
The police are there to establish the facts on behalf of the CPS, some of which will need to come from the AAIB as they are the only ones with those facts.
The CPS decide if the law has been broken based on the facts provided by the police and if it is in the public interest to charge anyone.
However, what happens if the AAIB during the course of an investigation discover that something has been done to the aircraft which was deliberate and therefore put the aircraft/pilot and public into danger (like cutting the brake line on a car, causing the brakes to fail). Do they then have to report the matter to the police and provide the evidence they have found in their investigations, or do they just make a report as normal. After all they are not there to lay blame but must have a duty to report such matters, thereby laying blame.
So if a pilot can be charged if he makes an error that could be deemed reckless, as he should have had the experience and knowledge to know better. Then the same could apply to maintenance were the ground crew should have the experience and knowledge to know better and therefore could be charged too.
After all it is now common practice to charge doctors for failures in noticing patients symptoms and warning signs that lead to their deaths. The same logical thinking could be applied in aircraft 'incidents'.