As well as the BOI members the Army has (had?) the permanent AIFSO office, which comprised an SO1 Engineering Officer with a WO Assistant, both of whom had done the AAIB Course, usually with 40 years plus of aviation experience between them.
PS. Not only could this accident have happened to any of the services, it already has on numerous occasions - sadly this is nothing new and it won't be the last.
PPS. Qualified supervising officers are essential, but when Mate A and Mate B strap in and lift off, all bets are off. If they do not have the discipline and rigour to associate what the Authorisation was with what they decide to do on the day, there is nothing the Authorising Officer can do.