Close unkind43.
Within the RAF and I guess the more senior Services, an airmen has the right to give 18-months notice to leave the Service enshrined in statute - The RAF Terms of Service Regulations 1985 or RAF ToSRs
(Statutory Instrument(SI) 1985/1820 subsequently amended by SI 1989/994, 1990/2374, 1997/231 and 2001/542). There are RM and Army ToSRs too. It is this right that is restricted by the quote you mention - however, the imminent national danger has a high threshold (ie only when Ivan, Fritz or Pierre is about to swim across the channel). (I seem to remember that 2 of the Services have stopped this happening in the past; but, I digress). This Notice as a General Right is different to PVR and it does not apply to officers.
PVR on the over hand has no statutory basis so the Service(s) can decide (and vary) the terms on which it is offered. Hence why, in the RAF, different cadres (trades/ranks within trades/holders of TQAs etc) have differing waiting times for PVR that can change dependent on the manning situation.