Look at it from a Police point of view.
Maybe there are officers with IDs airside also looking for the offender but that this officer has been placed to check anyone who hasnt obviously got a pass - maybe because its inside a jacket or its turned around by accident or, crucially, that they dont have one.
Also, think how easy it would be to get a generic Pilot outfit. And think how easy it would be for someone dressed like a pilot with a hi-vis to just walk around the ramp without being stopped.
Because you can guarentee that if someone is going to go to the lengths to enter the CP of BHX to attack the Copper Chopper then they will have taken measures to not get caught going out to, or coming back from the aircraft - you dont just breach a CP fence and attack a helicopter as a crime of opportunity done on the spur of the moment.