It is my understanding that driving a bus for leisure, ie a preserved bus, does not fall under either EU or Domestic driving hours as its basically being used as a big car - particularly so if its MOT as a class 5 and insured under a preserved bus policy.
Many a time I've taken my brothers preserved bus out during the day for a few hours then gone to work driving a service bus on a late shift - driving a preserved bus doesn't tire me out in the slightest, probably because there's no schedule to concentrate on, no passengers to deal with and is literally a leisurely drive!