Pub Social responsibility laws queston?

Hey im just wondering if anyone knows if a pub is withing its right to serve somoene at the bar, no questions asked, then for a bouncer to scan the public area after looking to I'D people whilst they are actually drinking? It seems to be a growing trend in pubs in the UK now, it happened to me last night, I bought a drink at the bar without being ased for proof of age, I was sat uptairs when a bouncer came over saying he was checking peoples I'D's. I was actually sipping my alcoholic drink at the time, this strikes me as a very pointless rule because if I ws underage, that pub would already have committed an offence by serving me. As it is I am 27, they have a poster up satting they operate a challenge 21 policy, I may not look 27 but i certainly do not look younger than 21! Any info would be graetly appreciated. Thanks