You raise a good point, I was thinking of connectivity in terms of routes between villages, but having easy access to a major A road without having to go through another town does mean it makes sense to mark it as a higher priority route. As an aside, although I am used to driving on rural Norfolk roads, this particular one is really bad in terms of potholes and maintenance. Perhaps navigation apps will take into account the smoothness tag when calculating longer routes.

I'll change it back to a tertiary now. Thanks for your help!