But that's not the plan is it?On the narrower roads that alck the visibility for safe, high speed driving, then 50mph is quite acceptable.
My understanding is that the NSL will drop to 50mph for all single carriageways and then local authorities will have to increase to signed 60mph if they can justify the case. I can only assume that "Trunk" roads (administered by the HA) will automatically be re-signed at 60mph. Although after a lot on A roads were de-trunked in the 90's (to move the maintenance costs to local authorities - sneaky one that) the amount of single carriageway Trunk road left is limited. After the de-trunking Somerset imposed a 40mph limit on a large sections of the A38 - even the long open sections between villages, this was purely to reduce wear and tear and maintenance costs (common sense did previal and they were raised again).
I can think of large sections of A and B roads where at 60 limit is perfectly reasonable (and as you say John large sections were it isn't). Something more granualar is needed rather than a simple blanket NSL reduction.
On the country lane front I can see something like road width working e.g allow local authorities to easily drop the limit to 40mph on any road less than 12ft wide.