Chug2, let's admit your suggestion, for the sake of argument, that the old system was flawed. If it was biased in favour of "their airships", they would be unlikely to invite the plods in. So it must have come from politicians. So what event, or process, gave them the ammunition they needed to get the hierarchy to let in the police. And remember, their participation did not do away with BoIs, it was additional.
Just briefly, hoping not to drift my own thread, in my experience BoIs were good, carried out by those who knew the nut & bolts of what they were investigating. They only got corrupted when complete, signed off, and handed to higher authority. All right, they aren't really complete until they have the CinCs remarks, but when I talk about "The Board", I mean the originators of the report.