My assumption was that the safety duties were Protection, Dispatch and passenger related duties after an accident such as actually going with the passengers off the train (if not needed to protect the train), possibly walking down the train to check everyone is out, first aid, making sure people stay on the train if that is safer.
In most companies, fire training for most staff is limited to getting out. In some firms, some people are trained in the use of extinguishers. First aid is limited to a person with a one day course for many workplaces, and the highest level required is a 3 day course (they shortened it from 4 last year).
GB, I think the Protection benefit comes from having CSR. My preferred option would be CSR whoever is onboard, and for that matter all lines to the higher DOO standard. But my preferred options can be very expensive.
Now I have been set straight, I don't see anything wrong with a ticket collector replacing a guard, so long as they are trained for evacuation, it is safe for the driver to do the doors and the safety authorities are satisfied that protection by waving light/red light/detonators isn't neccessary.
My preference, subject to safety issues, is for the doors to be done by the driver, leaving the second crew member to be at any point in the train as neccessary.