I would surmise that kilometres were ínvented' around the same time as UK miles.
We know that m in mph does not mean metre, because the metric unit of road speed is km/h, as you quite rightly say.
In Thailand, KILOMETRES are used, not pre used!.
So can you not deduce that 80 means 80 KPH, 100 means 100KPH and believe it or not, 120 means 120 KPH. Precisely.
Yes, on many roads now there is a maximum of 80 in the left lane, minimum of 100 in the middle, and maximum of 120 in the outside. Simple isn't it?
Then when you get a 100 sign, does that apply to ALL lanes or what?
The video shows the rear end of a Honda, and a quick glimpse of a yellow 'supercar'. How many were involved in the convoy?
Was it all the way from Rugby to Gretna?