BT aren't offering SDSL wholesale yet

That isn't quite true, they are offering it but it is very expensive and available only on a limited set of exchanges.

With an undertstanding ISP ( http://www.aaisp.net/multiline.html ) and a load balencing router at each end it is far cheaper to get 4 512/256 ADSL lines (resulting in 2048/1024) than to get a single 1024/1024 SDSL line.