Leaving the hay bales outside vs. barn storage has a lot to do with nutrient loss, and labor.
Understand. During baling you drop them in the field and then come back to spear them to trailers or to a barn. If this farmer is hassled for dropping them in the field as he bales (may not happen in UK , but it would in litigious US)then that would be a shame. Have baled thousands of round bales and on embankments, and there is a way to do it that doesn’t cause them to roll down hill.