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. Sri Lankan folklore: Perpetual copyright. Permission to make any work derived from folklore must be sought from the Minister in charge of the subject of Culture. This right is claimed worldwide. Works falling in this category are considered unfree on Commons and are not allowed.
2. Official text of a legislative, administrative or legal nature: No copyright.
3. Audiovisual work: 70 years from the date on which the work was first published, or, if unpublished, 70 years from the making of the work.
4. Anonymous works: 70 years from the date on which the work was first published.
5. Applied art: 25 years from the date of the making of the work.
6. Any other type of work: 70 years after death of author, or in the case of works of joint authorship, 70 years after the death of the last surviving author.