I always flash lorries in as well. They always thank me by flashing their indicators . Only happens when I am towing big trailers of course. Don't tend to hang about otherwise.What consequences?
The trucker that's just overtaken me is looking in his mirrors trying to judge when he's clear of my car / van / whatever. I aid him by giving him a flash, he pulls in and the truck that's behind him can carry on with his journey too. Everybody's happy. I don't go around flashing at just everybody, you know. In those particular circumstances it's actually an aid to road safety. 90% of the time the overtaking trucker will double-blip his tail-lights in acknowledgement.