Roger wrote:

Construction of a good proportion of our motorway network was actually
started before the blanket limit came into force and and I doubt if the
basic design concepts have changed since that temporary limit was
introduced so all motorways should be good for derestricted motoring.

Unless things have changd in the past couple of years, the design limit
for motorways was 120mph. There are however a few motorways I know of
where even 70 would be too fast given the sight lines. So if the
national speed limit were raised then there would need to be some areas
with a local speed limit.