As you point out storage heaters are typically hard wired in to a wall plate which has a fused switch in the wall. They are not normally plugged in via a standard power socket.
From a purely technical point of view it would be possible to disable temporarily the electricity to that circuit, replace the special wall plate with a standard plug wall plate and then use a Zigbee or Z-Wave smart socket to control the power to the storage heater.
However not only might you need a qualified electrician to sign off this alteration from a safety point of view but as @7andy points out there maybe special regulations governing how a storage heater is supposed to be connected meaning this approach might not be allowed.
Therefore the first step would be to consult an electrician or storage heater supplier for advice.