Well said DV.
You mentioned the Mull of Kintyre case, and that is a classic example of authorities conflating the roles of BoIs/SIs and FAIs/Inquests. There has been an Inquiry into cause of the accident, which has been set aside (in 2011) and not replaced with anything. But given the evidence that forced that decision, the obligatory Investigation into the deaths has yet to commence. Hitherto, the 'guilty' were known, they were dead, so no investigation was deemed necessary. (Itself a decision that assumed the Board had met its obligation to consider prior negligence, which it didn't).
However, the Scottish Fatalities Investigation Unit, part of the Crown Office and Procurator Fiscal Service (the broad equivalent of the Crown Prosecution Service) has gone further than the police or MoD. It has identified the crime scene (Directorate of the Air Staff in Main Building) and ruled in 2016 that this means the Metropolitan Police now have primacy. Unfortunately, as you have found out on Nimrod and Tornado, this is simply their way of offloading their obligations. Nevertheless, it was a 'brave' call to make. Perhaps he knew he was leaving! Maybe his successor will meet her obligations?