I've also just searched the BT website, for myself actually.
Less than a year ago I was renting a house, and specifically phoned BT customer services and asked for such a service. This is how I know how much it was.
I am now in the position of re-connecting a line in a new place, and am looking into this right now (I'm actually on hold to BT as I write this).
When I find out if the same service is still available, I'll report back.
Otherwise, misslideaway, I'd suggest the same as the above -- since you already have the telewest stuff, go with cable. The broadband is better than ADSL anyway.
Also, if you don't already have a BT line installed (i.e., no BT phone boxes on the walls), then you'll need to pay about �130 for connection.