Taking a further step back
"hands up who knows what the current military ID card looks like. I bet no more than 1% of the population" Well me for a start and certainly each one of the approx 450,000 members of the armed services (not including all those ex forces, family members etc). Not exactly known for its rarity value then?
Hands up all those who know what a CitizenCard ("The UK's Leading Proof-of-Age Scheme") looks like.
Hands up all those who knows what a 'Validate UK" National ID card looks like
Hands up all those who knows what a 'ProofGB" National ID card looks like
Hands up all those who knows what a 'Young Scot" National ID card looks like
Hands up all those who know what each one of the 15 regional ID cards looks like
How to publicans and shop owners know about all these different forms of ID? Simple really, they have sample copies sent to them by the licencing department for comparison reference. Just how difficult would it be for the numtys in the Licencing departments to suddeny realise that there is such a thing as the Military in this country and add a Military ID to that sample poster?
"Why should everyone in pubs be expected to recognise and accept forces ID when for a few quid he can do what the rest of the population is expected to do?" Well simple really. Why should he, he ALREADY HAS a perfectly valid ID issued by the Government, with his date of birth, already in his in his pocket.