You seem to have used a poor analogy. If you find money in the street and have a genuine belief that it would be impossible to reunite it with its owner you can't be prosecuted for theft if you keep it. (The law provides a specific defence under those circumstances).
A better analogy might be someone who books a theatre ticket but then decides not to go. He can't demand a refund. The theatre kept to their part of the contract by keeping a seat available for him to use. Similarly BSkyB might be able to say that they would have made Sky TV available, at your MIL's former address, if required. The fact that it wasn't doesn't give your MIL a right to a refund.
However I note that your MIL had the service disconnected. So it seems that she might have terminated the contract by so doing. If so (and assuming that your MIL wasn't locked into a minimum contractual period) the direct debit payments may have been taken in breach of the relevant banking code. Speak to your MIL's bank about making a claim under the direct debit guarantee:
http://www.thesmartwa...t-debit-guarantee.asp
Chris