Bleed radiators with no valve - is this valve near the boiler what I want?
The object you found in the airing cupboard looks like a gate valve, not an automatic air vent or air separator.  This valve might be for balancing the heat flow or shutting off a supply. I would leave it alone.
Check the back face of your radiators for a bleed valve. Sometimes they are out of sight or even under a cover facing the wall. It will be close to the top of the panel at one of the ends. If the radiator was mounted upside down then it might even be at the bottom and now useless.
Check the painted end of the cap at the top right side of the radiator, in case the bleed valve is hidden under several layers of paint.
If you really have no bleed valves then your best course of action depends on your plumbing skills. Your safest bet might be to ask for help from a plumber. I wouldn’t take any risks at this time of year. Little plumbing jobs can easily become a major problem if something unexpected happens.
Options are:

Temporarily turn up the speed of the pump. It might increase the flow    enough to push some of the air out of the radiator and back into the return pipe. This could move the problem elsewhere, or the air might find its way out of a vent. Remember to switch the pump back afterwards.
It might be possible to vent the air through one of the caps at the top of the radiator by temporarily loosening it little. Be prepared for it not sealing up again afterwards and leaving a leak.
It might be possible to change one of those top coupler caps on the radiator for a screw in bleed valve. This would require isolation of the radiator or even a full system drain down.
Replace the radiator.

If you do get any air out and you have a sealed system then it will need repressurising when cold back to its design pressure.