To clarify, in the UK a trunk road is defined as a road own and operated by the national government rather than the local government. On OSM, a trunk road is defined the highest performance road that is not a motorway. under the UK road classification system this means that a OSM trunk road is a UK primary A-road. with non-primary A-roads being `highway=primary` and B-roads being `highway=secondary`. 

hope this clears up the issue.