Trunk_link is also intended for channelised  (physically separated by an obstruction) at-grade turning lanes connecting the through carriageways/through lanes. The road itself visually looks like significant infrastructure unlike a tertiary road. I'm well aware that council C-road classification does not necessarily equate tertiary in OSM, if only because their own grading is frequently outdated. Am curious what prompted you to make the change in the first place. Was there something beneficial? Such as navigation routing agent behaviours?