the Guardian report says she did make the claim in the press, after FB. My guess is that something in the shop happened to upset her (there may indeed have been words about breastfeeding), that she blew it up into a FB rant and then didn't know how to back down when the press started phoning her, nor when the police came calling.
I have a twinge of sympathy for people in that situation; it's hard to find a way of saying "I exaggerated it out of all proportion, let's forget it." But I have more sympathy for Primark, facing some sort of Twitter storm on what appear to have been flimsy foundations, and even for the police sent out to interview what may have been an unreliable witness.
Still, perverting the course of justice is a serious charge, so there may be more to it than all this. I wonder if it'll stand up.