Sorry for the confusion - but we don't actually tell the IFR departure that he cannot depart, just let them know that they will get no radar service if we are too busy. Which, in a way, is no different to stopping an IFR into CAS if you are busy.
So all the TMA airfield approach sectors use "XYZ Radar" not "XYZ approach" so don't have to provide standard sep'n outside CAS?