In the past I have managed to get a force's discount travelling by rail - showing to to the guard and saying I'd forgot my railcard.
I can also claim to have been allowed to get my head down in the medical room (blankets provided) at Birmingham New St station after I missed my last connecting train to further North from there. After production of ID card.
Never tried brandishing it at a ticket office though - except when to back up a travel warrant.
Best Mod 90 (Do only army refer to it as that?) related memory though was flying into Heathrow on a NATO travel order. At passport control, in the queue..everyone shuffling about for their passports etc and I had a tatty bit of A4 in my drinken hands. Heard some twat behind me comment to his wife something along the lines of "watch this idiot get arrested". Showed ID to passport control who asked ME if I needed my travel order stamping. I replied something along the lines of "Well, you tell me..."
I awarded myself a quick glance at cunto behind me who was staring at the floor. I like to think he thought James ******* Bond was in front of him!