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.

1. This is an audio-visual and cinematographic work, and 50 years have passed since the year of its creation (that is, the work was made before 1974)
2. This is an anonymous or pseudonymous work, and 50 years have passed since the year of its creation or publication (that is, the relevant action was done before 1974)
3. This is a work of the government and 50 years have passed since the date when the work was published or first made available to public (that is, the relevant action was done before 1974)
4. This is a work of applied art and 25 years have passed since the year of its creation (that is, the work was made before 1999)
5. This is another kind of work, and 50 years have passed since the year of the author's death or last-surviving author's death (that is, the death occurred before 1974)

The Indian Copyright Act applies in India to works first published in India. According to the Indian Copyright Act, 1957, as amended up to Act No. 27 of 2012 (Chapter V, Section 25):

• Anonymous works, photographs, cinematographic works, sound recordings, government works, and works of corporate authorship or of international organizations enter the public domain 60 years after the date on which they were first published, counted from the beginning of the following calendar year (i.e. as of 2024, works published prior to 1 January 1964 are considered public domain).
• Posthumous works (other than those above) enter the public domain after 60 years from publication date, counted from the beginning of the following calendar year.
• Any kind of work other than the above enters the public domain 60 years after the author's death (or in the case of a multi-author work, the death of the last surviving author), counted from the beginning of the following calendar year.
• Text of laws, judicial opinions, and other government reports are free from copyright.