Do you happen to know if your ISP is giving you service over an 'unbundled' pair of wires to the BT exchange?
If this is the case, BT's involvement is merely to maintain the cable pair and the Exchange equipment will be provided by your ISP.
As you probably already know, the Upstream speed is always a fraction of the Downstream speed, typically my (wholly BT ) connection gives me 6,624Kbps downstream and 448Kbps upstream.
Alvechurch