I think a lot of the problem with flashing in HGV;s and caravans is an age and driving experience thing.
Although only 35, I was beside my father a lot while he was driving and payed a lot of attention with the usual why do you do that questions.
As my outfit is a good match and the car is more than capable of pulling it at over 70 I do try and stick at 60 but find myself overtaking other outfits and HGV's more than I am overtaken by the same. I will always "thank" the flashing driver by a quick alternate flash of the indicators or switch on and off of the side lights etc.
I will always flash someone in too if they are moving past quite slowly and will need to know as soon as possible that they can move back in.
Some other things my father did (doesnt drive now) or see on the roads:
Just after overtaking on a SC road, if it was still safe to do so, and he had good visibilty ahead he would stay in the oncoming carriageway so that any other cars behind also wishing to overtake can then do so (as any oncoming car would obviously hit my dads car before them so it must be safe)
Would also often see HGV's using their left indicator on a SC. This would be seen as a signal that the road ahead was clear for an overtake by following vehicles. Obviously I see it as been a signal to look out and see yourself if its safe to overtake rather than go blind.
Steve