In reply to:
I'm afraid, chaps, that companies or individuals are not allowed to bill unagreed amounts for services whether or not they are authorised or not.
That's not strictly true, but in any case you're missing the point. You're getting hung up on some notion of agreed charges, which has to be based on the provision of agreed services. The testing service isn't contractual nor is it agreed; you have no right to use it. BT would be well within their rights to bill you for the use of the service in order to cover their costs incurred as a result of your misuse. It would be up to you to prove that (a) you hadn't misused the service (difficult given that you had) and (b) that the costs were unfair in some way.
Of course we could take a different tack and point out that BT could pursue a criminal case against you for misuse of the telecommunications systems. Then take a civil action against you for losses incurred. Then probably cut off your line.
In reply to:
A parallel situation would be if I found your bicycle outside your house and borrowed it for a day, bringing it back of course.
Bad anaolgy: taking without consent is theft, whether or not you brought it back (or intended to). I'd then sue the pants off you for loss of earnings, grief, etc. etc. that I had suffered as a result of you depriving me of my bike.