I'd be the last person to support security at BHX, but I have to say that the reason for the BHX pass only (or escort) at the moment, is because of work to the terminal/departure lounge area.
Before work started (about 8 months ago), there was a 'non-swipe' route out on to the ramp from staff search.
Since work started, the 'diversion route' herds staff through domestic reclaim and some pax arrival corridors, all of which are protected with BHX 'swipe' doors ... hence the reason for the escort ...