Flashing your lights before overtaking these days would probably get you run off the road.
I use it regularly, I never have hassle with it, it's not a bad thing to do imo, provided it's not just a flash, I tend to hold it on till I'm sure the driver has seen me.
I flash artics to let them know its safe to pull in and that's about it
This is what i said earlier of it now being used as a 'thanks' or 'you go'
Its a bit similar to those people who put on their left indicator on for a couple of flashes as if to say 'its clear for you to overtake me'

I think not! I'll make my own choice if I want to overtake or not, although I appreciate what their doing as their heart's in the right place.