"going to the sales" is hardly a credible reason to start services running on Boxing day... The sales will still be there on the 27th, and the 28th and so on!! Boxing day is for chilling out, being hungover, relaxing after too much over-indulging ... if some sad act wants to go to the sales at 9am on the 26th.. they can get there using there own transport! If they don't have any... it will hardly kill them to wait until the 27th!
What about the transporting the hundreds of thousands of football fans who take to the road in the lack of a train service?
As rail companies have been privatised, the operators should be looking for opportunities to make more money and require less government support. If trains on Boxing Day have the potential to be packed, then run a service.
It also should be noted that a full timetable was once provided on 24th December. Now most services end around 6 hours early.