Robert Osbourne said introducing the movie that a movie was rated X because of shocking details (not sex) but the fact that it was rated X contributed to its popularity. What movie was that?
"Klaatu barada nikto"
That was The Creeping Unknown, ( quarter mass experiment ). This movie was originally intended as sequel but the author objected, so details were changed so it would not look too similar.
The previous highest rating in the UK was "H" for Horror but "X" replaced it because there were increasingly other reasons for restricting films (in the minds of censors, anyway).
On TV the title of the first Quatermass story had been "The Quatermass Experiment" but for the film version Hammer changed the spelling to "The Quatermass Xperiment" to emphasize the X certificate for publicity purposes.
"X the Unknown" was probably so titled partly for the same reason but also to suggest a relationship to "The Quatermass Xperiment" that they couldn't be too explicit about without attracting Nigel Kneale's ire.