Where does it say that it is illegal to ride with the flip-front up? Nowhere as far as I can find. I have searched all over for something definitive as laid down by the law, and there is nothing, just loads of layman-level discussion. True, you'd be daft to have it up much over 30mph (or whatever) 'cause the wind is gonna try and rip your head off, but law? No, not as far as I can see. As said, if you know better, PLEASE QUOTE SOURCE.
Even if flip-front helmets are tested with the chin-bar down and locked (and others have told me that the chin-bar itself is not crash-tested at all), that in itself still doesn't mean it's illegal to ride with it up.
And agreed, a copper telling you so is not gonna be enough - where is HIS source? Coppers do not know everything there is to know about the law.