Data protection and all that
Which is no doubt one of the reasons they didn't contact your HA (I'm sure it's a valid reason for withholding information your HA uses all the time)
Does the HA have an address for the (ex)tenant, would you release it to the DNO? If you haven't got an address what makes you think anyone else does?
If there has been any theft of electricity involved there is a procedure that is followed.
Installation of a pre-payment meter
if it continues
Disconnection by removal of main fuse.
If supply is illegally reconnected
Removal of main fuse and additional anti-theft equipment installed
If supply is illegally reconnected, this is getting into the grounds of dangerous activity, I've actually seen one property illegally reconnected using a claw hammer jammed into the cut-out in place of a main fuse, this hammer was easily accessible and live. Oh and there was a 3 year old in the property and the hammer was stolen!!
Disconnection of supply externally to the property (though often other chances are given before this last action)
You ask about criminal offences, illegal abstraction of Electricity is a criminal offence! Though it is rare that the police are asked to be involved.
What is stopping them from removing the supply to the next property?
What is stopping an HA from evicting tenants?
Who watches the Watchmen?
The Electricity Regulator
The Department of Energy
It's all governed by The Electricity Act & the Electricity Safety, Quality and Continuity and Regulations.
Other documents include the Distribution Code.
A simple Google would have turned these up and there may be a reference to them on the DNO website!
And as a matter of interest had the DNO involved the HA, what would you have done to stop the activity of the tenant before he possibly killed or injured himself or others?