I haven't driven on the motorway since the completion of my pass plus (about two months or so ago now, haven't found a need to go on the motorway again yet), but NOTHING irritates me more than people flashing their lights at me.
A few nights ago I was driving in the dark and I had some arrogant so-and-so right up behind me with his full beams on (it felt like it, at least). I was actually unable to look into my rear view mirror because it was so dazzling, even with the anti-dazzle glass on. This was on a normal main road, too. Also on an A road for a little while.
At the end of the day if you are getting up behind people like that and flashing your lights, you're most likely dazzling the other driver. YOU are therefore a dangerous driver and a threat to others on the road.
So don't do it, unless the situation calls for, as a previous poster mentioned. Why is that so hard to understand? Surely your driving instructor / whoever taught you mentioned that it is dangerous to flash people with your lights because it can dazzle them?
EDIT: & The reason most drivers flash me is because I drive at the 30mph speed limit which is found and enforced on most roads in my area. My insurance means that I cannot actually go above the speed limit, therefore making me a safer driver!