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.

1. it was subject to Crown copyright and was first published more than 50 years ago, or

it was not subject to Crown copyright, and

2. it is a photograph that was created prior to January 1, 1949, or

3. the creator died prior to January 1, 1972.