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.
Were you doing less than 10mph*? If so, he can overtake, despite the double solid white lines...
But somehow I think you were probably going a tad faster than 10 mph...
*Rule 129 of the Highway Code:
Double white lines where the line nearest you is solid. This means you MUST NOT cross or straddle it unless it is safe and you need to enter adjoining premises or a side road. You may cross the line if necessary, provided the road is clear, to pass a stationary vehicle, or overtake a pedal cycle, horse or road maintenance vehicle, if they are travelling at 10 mph (16 km/h) or less.