Its got to be said that courtesy isn’t what it used to be. Apart from the flashing and cutting in issues, I’ve noticed another trend recently.
I’ve pulled over to let someone onto the motorway/dual carriageway and they join and then sit on my inside and don’t let me back in. GRRRRRRRRRRRRRRRRRRR 
Happened 2 or 3 times recently. As someone said, its very tempting not to bother for the next one, but that would probably be unfair taking it out on someone that probably would let you back in. And unprofessional.