There no discounts on the broadband for taking TV. The only discount is £5 off broadband for taking calls - however even then this only applies to ADSL rather then fibre.
I would sugest reading up on BT Vision services and threaten to leave if they don't give you a good deal on the fibre, even if you don't have any real intention of leaving.
Note that in addtion to any "3 months 1/2 price" with BT you can also get cashback via places like topcashback
You might be able to get a discount on the sky line rental - people have had it for £1 per month recently, which is about £11 off your bill.
Sky Broadband Unlimited
Ashington (Northumberland) Exchange
Fibre due June 2012!