Be* is a bit eccentric. I completely forgot until jchamier's post that it uses MPoA (with no username/password required) rather than PPPoE.
Sky's "MER" system is pretty similar - and they're probably both more efficient than using PPP. Neither Sky nor BE needed to use PPP as their LLU networks didn't/don't support any other ISP connections - and they know who you are as your line is attached to their DSLAM / ISAM / MSAN port.
Whereas BT Broadband uses the BTwholesale network, as so you need to pass your ID so the wholesale part knows to connect you to the BT ISP, rather than say PlusNet, or Aquiss, or any of the other 100+ ISPs using BTwholesale.
James BT Infinity 2 19/09/2012 - Estimate 44.6/6.5 - Install 52/12 - Actual 46 / 8 Mbps
Huawei VDSL -> Draytek router -> Apple Airport Extreme -> Belkin Switch -> Windows/Mac/Linux/NAS/Phone
13 years of broadband - 1999 ntl:(512k/1M)/BTbusiness(2M)/Metronet(2M)/Bulldog(8M/16M)/BE(19M/16M)/BT FTTC(46M)