Hi Angie8m,
I know this is a bad situation,however, your tnt does have a responsibility to ensure that all their bills, utility and the like are paid, after all, he cant really live without electricity, you say he aint really using it at the min, but the weather has changed, and after all, he needs lights at night & to turn on the kettle !
Any utility has rights to access a property to inspect their equipment. If they do have to enter the property I would suggest its done with full consent as if they have to break in, which a warrant allows, their will be costs for lock replacements and if the tnt loses these keys what would you be left with.
At the end of the day if your tnt moves out, the debt will follow him as I am quite certain you would provide the forwarding address details, as you would know this for when you return any deposit/bond he has, and the benefit for you, yes there is one, is that prepay meters can be simply reset back to zero for any new tnts you move in, and you will be safe in the knowledge that they also cant run up massive debts by failing to pay their bills.
I would contact the tnt and collect any correspondence he has been sent concerning the warrant visit and get in touch with ScottishPower and explain you are the landlord and you would like to be present to ensure all goes smoothly.