I doubt whether there will be many punters using this bus service and here is the crass management of FGW▸ absolutely throwing money away running a bus service on a day when there are NO rail services to connect with, and that includes Cross Country and South West Trains as well as First Great Western, and when, no doubt, a day when the bus drivers will be demanding at least double time pay for their services on that day.
National Express (the bus company, not the train company) operate a normal service on Boxing Day too, y'know...