This is just the southern cross section in question - the distances between the junctions there are huge!
The speed limit on the Southern Cross was 60mph on the day it opened, so it must have been a planning permission thing.
OP the 120 km/h limit for motorways is just a default, not a requirement. Road authorities are free to set special speed limits on motorways just as on any other road.
It was - noise concerns as I mentioned.
If you travel the whole length of the M50 you'd take 4mins longer travelling at 100km/h than at 120km/h. Hardly a big issue, but time savings of 4mins have been used to justify spending millions on other schemes.
I worked on the M50, and at the time one of the reasons given for the lower speed limit was higher capacity. Traffic can travel closer together at 100 than at 120, so more vehicles can physically fit on the road. In addition to the narrower lanes, noise and junction spacings I don't think that 100km/h is a bad compromise.
It might be 100kph but I can't remember ever having done 100kph on it.