Personally, if I was a customer, if I had the choice of being pointed in the direction of a local shop there-and-then or having to wait a few days til they could replenish their stock of the magasine, I'd WANT to be pointed in the direction of another local shop. To be honest, it's not like a customer is going to stop shopping at Sainsburry's and, forever more, do their grocery shopping at a news agent!
That said, I definitely agree that from Sainsburry's point of view, B is not the best answer.