Call up sky, tell them you're thinking of leaving, those magic words should get put through to the "retentions" team and the floor is all yours for negotiations.
Remember the old business adage, it costs ten times more to win a new customer (marketing costs etc.) than it does to keep an existing customer loyal.
(Might help your cause if you do a little homework before planning and see what the cheapest package others are offering is .... even if they don't serve your area, e.g. Virgin cable ... being able to quote actual lower prices at the retentions team will help more than just haggling..... because they may well be able to offer "competitive discounts" that do what they say on the tin.... they are there in order to win you away from the competition).