You use quadrantals when you are flying above the transition level (which varies around the UK) and outside controlled airspace. You 'Must' use quadrantals if you are IFR, unless you are asked to fly at some other level (Eg. under a procedural service) or are manoeuvring. You can fly quadrantals (and the CAA recommend) if you are VFR.
As you are OCAS, IFR is simply in your mind, you don't need to tell anyone - nor does anyone generally care.