You may believe the folklore that that's the day the Brits give their help their Christmas presents. That may have been true, oh a million years ago, or maybe even now for a Very Small Number of Brits who have 'help.' For the rest of the country, though, Boxing Day is when you have a hangover from the excesses of Christmas Day. Here's what I recall of Christmas Days when I lived in England.*
Hamish and I were visiting our friends, Bunny and Ray, somewhere in the northern reaches of England. I can't remember the actual city, but it had to be one where there was a rep company, because Ray, like Hamish, was an actor. And Bunny, like me, was a wife who worked. I've arrived on Christmas Eve, because I had to ply my secretarial trade that day. My boss, the porcine Mr. Platt, in his best fatherly fashion, had dragged me and my huge sacks full of Christmas gifts in a taxi to the train station. Mr. Platt was porky; he was also a Republican; and he was also very kind.
When the train chugged it's way north, I drank coffee and ate a stale British Rail cheese sandwich. The former was poured from two pots, one containing coffee and the other hot milk, at a great height, say 2 feet or so, by the porter. Without spilling a drop--quite a feat. The latter was, as were all British sandwiches then (and now?) composed of a single slice of cheese between two buttered slices of white bread. Yum. If you were lucky, there wasn't any green on the bread.
When the train pulled into the station, Hamish and Ray and Bunny were waiting for me. Or maybe it was just Hamish and Ray. Or perhaps just Hamish. I can't remember because, frankly, the coffee was the last non-alcoholic beverage I drank for the next 36 hours. Each of us had laid in a store of our Christmas drink. I don't remember what the others had, but mine was a pitcher of gin martinis, light on the vermouth, kept chilling the refrigerator. We drank. We ate. We watched endless variety shows on TV. And we drank some more, and the next day, Boxing Day, we had to drink again, because without Hair of the Dog, none of us could put one foot in front of the other.
The cast of characters changed over the six or so Christmases I spent in England, but the activities never did. We drank, we ate, we watched bad TV, and on Boxing Day we recovered.
*Which was, admittedly, some time ago, so maybe they have become more civilized since then.