I was overtaken by an AA instructor last night, on his own in the car.
It was a textbook perfect overtake: all four wheels over the centre line, not too fast, no harsh acceleration.
Apart from one thing: the centre line was a double white solid line.
Luckily for him (and me) the car coming towards him wasn't doing the full 60 mph allowed on that road. Also luckily, the car following him had the very good sense to see what was about to happen and hold back. So he was able to slam on his brakes and tuck back in behind me.
When the oncoming car had passed, he overtook again, and as he did he made strange hand signals at me.
The cars following him didn't overtake until there was a safe place, unlike the so-called professional instructor.