The presence of gravy. In the North, you get whole sections of aisles full of every kind of gravy imaginable! When I briefly lived in the South, there were whole shops that didn't sell it. Imagine my relief when I found some in hidden in a back corner of a tiny Co-Op, like some illicit product!
Also, you can get 'pudding and chips' from chip shops in the North. We were amazed at the blank looks down South, and the presence of mushy pea fritters instead...