Its bizarre isn't it, I was on the M4 I think it was driving towards Bristol and I overtook an old Audi who was pootling along on the inside lane, I just pulled out, 5th gear, and floored it and pulled back in and slowed a bit. I looked in the mirror and the Audi was slowly catching up, he pulled out, pulled up next to me, was a guy about early twenties I think, laughing his head off giving me the thumbs up and saying 'nice car' across at me. I overtook him again and he gave me a wave as I tore off...think his old green audi was falling to pieces at this point
It restores your faith in humanity when that happens instead of someone cutting you up, tail gating or generally driving aggressively trying to cause an accident.