1. This is a photographic work, audiovisual recording, motion picture, sound recording, or broadcast work, and at least 50 years have elapsed since its first publication or it was not published within 50 years of authorship.
2. This is a work of applied arts, and at least 25 years have elapsed its first publication or it was not published within 25 years of authorship.
3. This is a work by a pseudonymous or anonymous author, and at least 50 years have elapsed since its first publication or it was not published within 50 years of authorship.
4. This is a work other than described above, and
   1. Its last author died at least 50 years ago, or
   2. Its last author died before publication and at least 50 years have elapsed since its first publication, or
   3. The author is a legal entity, and at least 50 years have elapsed since the first publication of the work or the work was not published within 50 years of authorship.
5. This work was created or commissioned by the Thai Government, and at least 50 years have elapsed since its first publication or it was not published within 50 years of authorship.

This is because it is one of the following:

1. It is a photograph taken prior to 1 June 1957; or
2. It was published prior to 1974; or
3. It is an artistic work other than a photograph or engraving (e.g. a painting) which was created prior to 1974.

HMSO has declared that the expiry of Crown Copyrights applies worldwide (ref: HMSO Email Reply)
More information.