In my experience most people don't even noticed they are being flashed or being an idiot in the middle lane (or any overtaking lane to be honest) they are completely oblivious to driving and what they are doing. Whilst on the M4 on Sat, cruising at a steady 70ish there was someone sat in a mostly empty motorway in the middle lane, a van went up behind the car and flashed as I came up to them and passed in lane 3, I then moved over to the clear lane 1 in a single move, the middle lane person didn't understand. The van overtook them and also moved to lane 1, closely to the car, again they didn't understand and stayed in lane 2.
I've given up caring about those idiots now mostly and just let them get on with it, the police don't care either which makes the fine etc. pointless.