I was certainly Gazetted as an APO so I would assume that that is the substantive rank.
Thet Wiki article referenced above is somewhat disengenuous claiming that APO is the lowest Commissioned rank in the British Armed Forces given that (certainly when I was an APO) it was directly equivalent to the Navy's rank of Midshipman and the Army's 'Second Lieutenant On probation'.
The idea that it is somehow equal to Officer Cadet is also not true. At Cranwell (although we were treated with equal contempt and wore the white flashes) we were referred to as 'Student Officer' whereas the direct entrants (even if they had degrees) were referred to as 'Officer Cadet'.