'fraid you are mistaken on two counts:
Firstly, On open ended (permanent ) employment, the notice period is as determined in the terms and conditions you sign. For example, I have a six month notice period if I resign - and most of my staff are on 3 months notice.
Secondly,the RAF doesn't offer you open ended employment, they offer you a fixed term contract, which can be extended to further fixed terms , by mutual consent, depending on a number of criteria. Essentially, when you PVR, you are not resigning as a permanent employed would, you are breaking the terms of a contract you signed up for - therefore the employer can either hold you to the terms of the contract - for example, refuse the PVR request, (which has happened in the past) , or determine the terms on which they will allow the contract to be broken, , such as time period before release, payback of investment in you etc...
If you want to look up the law on it, the first one falls under FTE law (full time employee) and the second under employment law for FTC (fixed term contractor)