Ah, apologies - my fault for skipping through to the latter pages of the thread. The same principle applies, I think - they know (rather arrogantly) that people will keep coming back, so they are not that fussed about people leaving.
Virgin (and Sky in the past) occasionally offer me upgrades...
Done.
On the subject of leaving Sky (or Virgin, or whoever), they probably aren't too bothered because they know you'll be back. I was talking to the guy that installed our Virgin kit, and he said that, like car insurance, etc, a lot of people shop around once they are free of their contract...