You have what is called an EO or Exchange Only line (as do I). I regret to say that the chance of your getting a decent connection in the near or middle future is pretty low as enabling EO lines is very low on BT's priority list and being in London there is no BDUK funding available to help.
You can check whether you are on an EO line or not using the BT whoesale ADSL checker
If you are on an EO line you will see something like:
Telephone Number 0207587xxxx on Exchange VAUXHALL
whereas if you are connected via a cab you will see something like:
Telephone Number 0207582xxxx on Exchange VAUXHALL is served by Cabinet 18
Edited by MCM (Fri 29-Nov-13 01:43:47)