Sky are fine as long as you are upgrading or a new customer. Existing customers have to threaten to leave to get anything basically, and then if you do decide to leave they won't let you leave when you want to!
I was tied into a year's contract for HD when I thought it was only a trial! Because they wouldn't let me cancel I said I wanted to close my account entirely with Sky on the day my contract ended but was told I couldn't give my month's notice until after the Contract had ended, making me pay them for another month. I refused and cancelled my direct debit on the day my contract ended. They then threatened me even though they stopped my service the moment the direct debit failed and they couldn't get their money ...... they wanted a whole month's money even though I had no service!!! Their debt collecting team are basically like the worst kind of debt collectors imaginable and treat people with no respect whatsoever.
I wouldn't touch Sky with an extremely long bargepole, and they dropped the amount they said I owed when I threatened them with legal action.