It's been happening in the U.S. For years, your in uniform,fit for work,going to your place of work, you will be listed on the paperwork as crew member..... Indeed I'm sure in the unfortunate case of pilot incapacitation there may be something in the ops manual that would suggest "A positioning pilot, fit to fly may occupy his/her seat in order to continue safe operation of the flight..."
Also,I would say that an airline that has 300+ aircraft, 2000 flights a day etc is not really in the habit of doing something illegal in the eyes of the DFT Numerous times EVERY DAY, for the last 20 odd years! If the authorities/security had a problem with this then it would have been addressed by now don't you think.......