Some may be.
As today is Saturday, tomorrow will be Sunday and cannot be a Bank Holiday. If you are referring to Monday, which is a Bank Holiday in lieu of Boxing Day falling on a Saturday, then it is likely that most supermarkets and some larger stores may well be open.
If you have a specific store in mind, I would suggest that you do a Google search for the store in question to see if they have a website that will give you their holiday opening hours.
