Three months ago I phoned to cancel my sky contract. I was offered half price subscripiton for 3 months (which I accepted as well as downgrading to the family pack)
Couple of days ago the three months was up, so I phoned to cancel - and was offered half price for another 3 months - which I have taken up.
Not sure how long they will do it for, but it looks like if you phone them up and tell them you want to cancel, you can halve you subscription costs.