If you're currently paying full price for your broadband with Sky you just need to go through the process of 'cancelling' online, and it will come up with an exclusive offer before you actually process the cancellation.
As far as I know it brings Broadband Unlimited from £7.50 down to £2.50 for 12 months. I'm not sure what it discounts on other packages.
But you need to do the following:
1. Log in.
2. Hover over MySky
3. Click 'Review Package'
4. Scroll to the bottom and click "thinking of cancelling broadband, phone and line rental".
5. Then you need to choose reasons why, which is "going to another provider", and it's "due to costs", and select the other provider as being BT.
At that point an offer will come up which you accept. It does come with a 12 month re contract to the broadband, btw.
If you're in contract it will advise you do call as far as I know.