We used to get airlocks in the DHW system whenever it was drained down (too many pipes horizontal ). Remedy was to connect the hot and cold feeds for the washing machine and let mains pressure water back fill the pipes as soon as a rumble of air bubbles could be heard in the main cold water tank or water came out of the overflow pipe then it was turned off and system worked perfectly.

I suppose you could do something similar with a hose connected to one of the drain off cocks but you would need to secure it tight on the drain off cock or it could be water, water everywhere. The other thing to watch is the header tank as on CH systems they are quite small and soon fill the overflow should deal with it but better not to take chances.

As for why your header may be full of water it depends where the turn off valve is. If in the supply pipe to the ball cock then the tank will empty. If on the downfeed pipe header to CH system then water will be retained in the header.

Richard