Make a formal complaint in writing to Sky and then with the relevant ombudsman if you have to.
When you took out the subscription they should have sent you the terms and conditions that applied. I think with Sky, you sign up to a minimum subscription period of 18 months and in the event of cancellation, you have to pay the relevant amount. When you phoned them, they should have explained this. If they did not, then you have reason to complain.
Once you have submitted the complaint, they should put collection on hold. Send a copy of the complaint letter to the debt collection agency, advising them that as the matter is now in dispute, per OFT guidelines, they should put all collection activity on hold. If they do not do so, they will be reported to the OFT.
Hopefully Sky will resolve for you. You might have to pay something, but may be able to negotiate a reduced amount.
My experience with a relatives debt, is that Sky do not stop trying to collect. My relative left the country owing less than £40 and they were incorrectly traced to my address. Six years later they were still employing a debt company to chase for payment. So don't ignore. Make the complaint and try to resolve with Sky.