There never were any "plans". The introduction of the name was merely a decision by the Ministry of Transport to indicate that the roads in question were now controlled by the government, not the local county council. That remains the case to this day.
As an example, De La Warr Road...
The "Folkestone to Honiton Trunk Road" was the name given to what is now called the "South Coast Trunk Road" when the Ministry of Transport (as it then was) took over responsibility for maintaining and repairing a series of local roads that had previously been looked after by the County...