"However if you call Sky's bluff and actually cancel Sky TV at least, Sky are likely to ring you within a few days of you cancelling and offer a deal."
By which point you've had Virgin installed and no longer need any of Sky's services.
Sky tried calling me 4 times within the first 10 days after I called to cancel. Unfortunately, they kept calling during the day when I was at work and despite my wife telling them every time
that they would have to call back in the evening (and the caller acknowledging that they would), they never did! They just called back in the day again. Way to lose customers Sky!
Whilst I agree that Sky have got wise(r) to this kind of practice, it's a risky game to play whichever side you are on. Brinksmanship at it's finest.