Lower speed limits are imposed at roadwork sites for the safety of the workers. If the limit had been reduced from 50 to 30 it was for a good reason. If you were cought simply pay the fine and get on with life and learn from your experience.

If resurfacing is of the spray tar/throw down chippings method then the 20 limit, posted by cars on lamposts etc, is advisory.

It's purpose is to avoid damage from flying chips and to mitigate reduced braking action on the loose surface. Such limits are widely ignored and the SC van will be monitoring the normal limit of 30 40 or whatever.

A formal temporary limit, such as that in force round M/way works requires an order from highway authority and will be posted by proper statutory signs, albeit mounted on trestles.