Even in our own kitchen, if I am looking in the fridge for something, Lucas does not reach around me, bumping me, to get what he wants and can easily see without acknowledging me. At the grocer's, if someone is clearly looking at every salad dressing making a decision and I know exactly what I want and where it it is, I say, "Would you mind if I jump in here to grab what I need?" "Thank you.
It may be a public store, but OP was at that spot first, needed a bit of time, so no matter what, the other couple should not have behaved that way. They did not know what they wanted! If they had, they should have stated it.
I would have politely defended myself, saying,"I will look through these real quick and be out of your way. Thanks." That kind of matter of fact, forthright stance usually makes people back down. If they hadn't, I don't know what I would do. Probably walked off because the next step would be physical.
I see nothing wrong with the OP's actions. A little weak-spined, maybe, but that is her decision.