They were exceptional circumstances. Gridlock. And all it turned out to be was a short run of temporary roadworks lights. Unfugging- believable!
I'm sure there was increased volume going that way due to 10 & 11 being closed on the M25.
As for 20mph zones - I had no trouble sticking to 22mph.
It's wrong to look at impact speed and compare with free traveling speed. there is no doubt that hitting a person at 20mph is going to result in less injuries than hitting someone at 30mph. But the evidence suggests that 20mph zones don't work, because pedestrians and drivers pay less attention to the risks. No rational human would cross a 40mph road without a really good look, but with 20 zones, people don't see the risks. Head in phone, step out.