I had also seen that previously (and other lawyers arguing the same). It it your intent when you leave home that is important, not what you do while you re out. So if you intend to go for essentials (food/medicine) but on the way back decide to stop off at an off-licence for a box of their finest wine, you are not breaking the law. However, if you leave the house just to go to the off-licence, you are.
Groceries as well usually, because most of them are corner shops. Regardless, they have added off licences to the list of essential shops.
Seems fair enough. Food is classified as an essential item. There are no sub-categories mentioned on the government website. Social distancing is the important bit.