Until the end of June (I think) BT will install a line for £29.99 if you agree to be on one of their call packages for 18 months. See:
http://www.serviceview.bt.com/list/p...o/SNL21-08.pdf
Or if you've got Sky and no BT line, Sky may offer to credit your account for up to £125 if you pay BT and take Sky Broadband.