I would, however, take issue over your description of the finding by the AOC, in which the AOCinC concurred, as a "Legal Decision".
The finding of the BOI, including the remarks of the AOC and AOC in C, will be an administrative finding.
I would like to take this thought process one-step further. The terms of reference for the BOI would have been clear to all parties involved and ‘should’ have led - as Cazatou mentioned - to an ‘administrative finding’ at the end of the process.
Did not Day go outside his remit when he sought legal advice prior to him submitting his ‘administrative finding’ to his AOC-in-C? Day, under questioning, places much importance to the legal input he received in coming to the conclusion he did. Surely interested parties have a right to challenge that legal advice which was - allegedly - pivotal in finding Tapper/Cook grossly negligent?