Annoyingly, there's no hard and fast answer to this. Some shops will stay open over Boxing Day while others will close and it depends mostly on your location. For busy high streets, such as Oxford Street, you can expect most retailers to stay open over Boxing Day.
Outlets in retail parks will mostly stay open but some supermarkets have taken to closing their stores over Boxing Day in recent years. Aldi, Iceland, Waitrose, Sainsbury's, and Poundland are some of the bigger retailers that have closed their stores over the past few years. With the pandemic passed, however, it could be that more shops are open for 2022 so it's definitely a good idea to check in at your local store's website before you head in.