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.

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.