I don't think it's fair to single out one Group Captain or one base for that matter...but this rough explanation will take a minute or two..
Once upon a time a very high percentage of jobs on an RAF station,.aircraft maintenance, cooking, bottle washing, medical centre tasks etc etc were done by enlisted personnel...on paper those individuals were available for use 24/7, so e.g. after a shift at work fixing aircraft or serving up food they could and were on a routine basis rostered for extra duties such as guarding the station gate or standing by to respond to security alerts...the Officers would also be rostered for an Duty overnight..
Come the back end of the 80s it became fashionable to contracterise (civilianise) lots of engineering and domestic functions so the number of enlisted personnel reduced....and since the civilians couldn't be rostered for stuff like guarding the enlisted bods who remained got screwed for more extra duties (did wonders for retention) and it became accepted that there simply weren't the personnel to maintain guarding levels...the increased number of overseas deployments also brought personnel
issues.
End result, there had to be fewer bases (Brize now does the job once done by three stations) and less personnel to do the proper military stuff such as guard the assets...the UK got away with it (not at war etc ).and the public didn't notice, until today.
Brize has it's vulnerabilities due to it's role and the aircraft based there are so large they mostly can't be locked away and sheltered overnight, but I suspect similar to what happened last night could have happened at quite a few RAF stations...so I.hope we don't simply see OC Brize getting all the blame for this.