Yes technically there is no reason why an iStore cannot be on a controlled load circuit.
Note that technical support during normal business hours can be limited unless there is a manual override of the circuit available.
Utilising the built in timers is typically a more flexible way to restrict the operating times to those desired.
It is not advised to attempt integration of the iStore built in timers with the Offpeak circuit as the grid removing power at varying times may interrupt the heating cycles from the built in timers. Removal of the power will leave the iStore in the state it was in at that time, for example if the unit was in the OFF state when the power was removed then it will remember that and be in the OFF state when the grid turns the power back on. The iStore will not switch back on until the start timer is reached and hence may not heat at all during the grid power supply time.