For the majority of Retail, Boxing day is and shall always be 26th December regardless of what day that falls on, the 28th had to be made the bank holiday alternative for boxing day for the full time workers not in Retail that have every weekend off in order for them to have the minimum required number of bank holidays available to take.
It all works out in the end, the 26th December falling on a Saturday would be the same as it falling on one of your days off during the weekday. In terms of time off everyones on an even keel. The only part which is different is that time off for Retail Workers don't consistently fall on the weekend, but that's been the case for decades and is widely accepted as part of the Retail industry.