If it's unusable, tell them and say you will stop paying for it as it doesn't live up to their advertising. Your rights are protected outside the contract. Threaten to complain to Ofcom if they resist. They'll let you leave and give you the MAC code.
I'm in London and my exchange has Sky LLU support. Coincidentally I live in the same postcode as Sky HQ so any problems, I'll talk face to face with them as I have some contacts.