You can never be "guaranteed" any given intake. If you defer in time and there are still spaces (both on the CC and PCCBCs) then great, but the joining instructions all make references to the fact that if you fail PCCBC etc. then they can't guarantee they'll have room for you on the next intake.
As for how full CC103 is, you'll obviously as mentioned have to call OCAC, or your ACA may well know. Why the deferral?