Responding to chilly's diary entry ...

As I read the Highway_tag_usage page in the Wiki at osm.wiki/index.php/Highway_tag_usage, highway=trunk doesn't mean a trunk road with the legal meaning of trunk road in the UK. The Wiki page says to use "highway=trunk" for "primary A road (green signs)" and to use "highway=primary" for "non-primary A road (black/white signs)", a bit counter-intuitive, but that convention does seem to have some advantages:

1. the renderers show "highway=trunk" in green and "highway=primary" in red, which matches the colour most paper maps in the UK use for primary routes and non-primary A roads, respectively, and

2. I'm not sure it is possible to determine trunk roads (in the legal sense) from any non-copyright sources. The original legislation is Crown copyright (perhaps it is out of copyright now), and the trunking and de-trunking orders that amend trunk road statuses are also Crown copyright.