Only leaving your home for essential purposes is law
Yes, but that law also makes it clear that it is legal to go out to exercise, meet one other person, buy food, seek medical attention and several other reasons.
Of course, most people could cope quite well never leaving the house at all. We don't need to go for a walk, it's just a nice thing to do. No-one's going to die within the next few weeks if they don't go for a walk. Do you really want to be in a situation where you cannot actually leave your house at all?
Similarly, many people will have food of some sort in the house, even if it's not what they;d choose to eat right now. No-one expected to be down to the last out of date can of soup and unidentifiable tub of brown in the freezer before they go out for more food, so its clear that the term 'essential' is subjective and it seems that there's only the Derbyshire Police force who think these women did anything wrong, or else we wouldn't have the CC of another police force stating on local radio that he was happy for people to drive 10 miles to the beach.