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 1975; or
3. It is an artistic work other than a photograph or engraving (e.g. a painting) which was created prior to 1975.

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

The Malaysian Copyright Act 1987 applies to works first published in Malaysia (details). The periods of copyright are as follows:

• For literary, musical or artistic works, copyright subsists for 50 years after the death of the author. Copyright for unpublished works subsists for 50 years after the publication. For anonymous or pseudonymous works copyright subsists for 50 years after publication unless the author is made known.
• For films and sound recordings, copyright subsists for 50 years after publication. For unpublished sound recordings, copyright subsists for 50 years after the fixation. For broadcasts, copyright subsists for 50 years after the broadcast is first made.
• Works by the government, governmental organisations and international organisations are subject to copyright for 50 years after publication: sections 11 and 23.
• Regardless of the above, the texts of laws, judicial opinions, and government reports are always free from copyright: section 3 (definition of literary work).