I'm afraid your stuck with £40 a month mate. No ISP will be able to undercut BT because they rent the ADSL from BT and then bill it on to the 3rd party (you).
If Isp's did try to undercut BT then BT would simply increase the prices they charge the ISP so that they werent able to.
As davidh says, cable is cheaper at £25 a month and is also better because the "up" speeds are faster. You'll have to wait until cable move into your area m8.