Taking over a telephone service without the owner's knowledge or permission is known in the trade as "Slamming" and there are very specific and strict laws and guidelines to prevent it happening. OFCOM or OTELO would take a very dim view of any provider doing so and they have a history of raising quite hefty fines to those caught doing it.
That said, Sky are one of the better companies that do not perform Slamming. The advice given above is sound and you should give it a go before getting on to the regulators. You also need to keep some written evidence of your complaint, so I would suggest following up all calls with emails or letters. A hassle, but you need to have a leg to stand on if you intend to go to OTELO or OFCOM.
Suggestion; when calling Sky do try your best to keep on a tight focus and do not get angry with the person you are speaking with. They are all limited in what they can actually do there and then. In any event it is going to take time to put right, and you can and should seek compensation for your energies spent, possibly after the event. I would suggest that you prepare a list of your wants and desires before you make the call, that will help you keep objective and on focus. If you get nowhere with the person you are speaking with then ask them for the escalation route, the manger or senior manager you could discuss this with. Be firm but also completely polite - you can always go kick the dog or cat after you get off the phone. I know this will be hard to do but it is really all in your voice as to how you get dealt with. Try smiling when you are speaking, crazy I know but it actually changes your tone to the person at the other end, helps them empathise with you. There's some real trick-cycling on the go and I don't know how it works, just that it really does!! I've had some really difficult situations to resolve, smiled my way right through the call and had the person at the other end not able to do enough to help me!!! Really