In fact a few years ago BALPA agreed a protocol with the DfT that commuting and positioning crews could use the staff search areas subject to certain caveats. From memory I believe that one could only do so within 24 hours of a duty and if requested would need to show your roster if asked to do so. However, when I tried to do so I got a "sorry mate, we ain't heard about this". At the time I emailed the AVSEC chap at BALPA and he said this airport had signed up to the agreement.
Thank you BBK for your insightful post. Especially re the arrangement with BALPA.
hunterboy, commuters will be well aware of all the rules regarding commuting in uniform as laid down in their company manuals. There really is no need for you to remind them that misuse of airline ID can get them in trouble, let alone arrested or suspended. Do you remind your doctor to wash his hands after you?