British needle exchange specifics

[TheDave]

SWIM's been to the enormous multi-floor chemist type place in his town, and asked where the needle exchange is, and was led through a tiny wooden maze to a little hatch where he was told "this is where you would go, knock on the hatch, tell them what sort you need" SWIM's researched what type he needs, around 28 gauge, the one with the orange cap he believes will fall into this category.

So what does he need to say when he knocks on the little hatch? "I'd like the one with the orange cap" or does he need to give a gauge number? SWIM is 100% inexperienced at this and is aware that turning up and asking for something he's read on the net that has no bearing on what they're expecting him to ask for might highlight the fact that he's not a junkie who's only other option is using dirty needles of the street but a curious studenty-type hoping to try it for the first time with his friends.

Any details of the specifics of what to do here? I'm talking about needle exchanges in the UK specifically.