If you are looking at infinity it offers 80Mbps and not a 40Mbps option. This is at the same price as the old 40Mbps option way back when infinity began.
There is the £18/month product with 40GB of usage which is limited to upto 40/10 (or 38 / 9 as BT advertise).
The unlimited usage is upto 80/20 (or 76/19 as advertised) for £26/month
Both prices don't include line rental or any special offers.
James BT Infinity 2 19/09/2012 - Estimate 44.6/6.5 - Install 52/12 - Actual 46 / 8 Mbps
13 years of broadband - 1999 ntl:(512k/1M)/BTbusiness(2M)/Metronet(2M)/Bulldog(8M/16M)/BE(19M/16M)/BT FTTC(46M)