Unless it is/looks serious I don’t stop. Too many people stop for minor bumps just to be nosy.
I did a first aid course on my last DCPC renewal, although I wouldn’t recommend the training company as they completely ripped me off.
You can also do other potentially useful things for DCPC - I also did a 1-day forklift refresher to keep my ticket up-to-date.
As for stopping/directing traffic - no, you have no legal authority, but my experience is that most people will appreciate that you’re doing a useful job and comply with the directions. That’s certainly what happened when there was a collision (car vs pedestrian) on a road near my house. An ambulance was already there dealing with the casualty, so I just directed traffic - having one person “in charge” allowing blocks of traffic through at a time means that you get a higher overall throughput of traffic compared to each pair of drivers “negotiating” on a one-by-one basis, so people experience less of a delay overall.