You don't generally need antifreeze in a central heating system. You do however need a corrosion inhibitor. The only one I have used came from screwfix but there are several on the market with varying qualities.
The bottle should tell you how much to use and you can put it into an empty system through any compression joint. Open the joint, pour it in and then close the joint again before filling the system.
fernox central heating inhibitor is the choice of the professional. easiest way to add to the system is via the F&E tank (if you have one) or via a towel radiator (with valves closed BEFORE refilling system). alternatively you can get concentrated inhibitor in extruder tubes that you can apply via bleed valve on a radiator.