In the Thames Valley Police case referred to by Chug (Nimrod XV230), the DCC claimed they could not proceed as there was 'little chance of witnesses coming forward'. She was sitting on witness statements, and replying to the letter that had submitted them. Not unlike what I describe above, but this time it is the HSE's principal investigator.
Airpolice, a complaint has been made, in May 2017. That is part of what the P&CC has instructed the CC to address. From other threads you will glean there is far more to this. Formal complaints have been made in a number of cases. Most police forces have not replied. Some just pass the buck. For example, Police Scotland (and Strathclyde Police before) acknowledge offences were committed by MoD/RAF on Chinook ZD576, but say it is for the Metropolitan Police to investigate as they were committed in London. The Met simply don't reply. Too busy tax collecting.