The Amsterdam orbital motorway has a restriction on HGVs during rush hours. As HGV drivers have restrictions on the hours they can drive it seems a good idea by the Dutch authorites to decide when they have their rest period. Not sure it would help the M25.
They draw lots. The same way as the FIFA & UEFA draws; plastic balls.
Sadly, I am old enough to remember them doing it in 1990 to decide who came second in England's group. It was between Ireland and Holland.