Which speedtest have you got the results from?
I mostly use speedtest.net and the bt wholesale test at http://speedtest.btwholesale.com/
I used to use http://myspeed.visualware.com/index.php
as it gave very reliable test results. A few months ago it starting returning very poor download results, and in the last month or so it has given bad upload results as well.
I found the old BT retail test (now dead) gave the most incorrect upload speeds. The new BT retail test appears to be just a shim on the wholesale test; complete with a very confusing report claiming to be sync speed but actually being a guess of sync speed from the IP Profile.
I stopped using the thinkbroadband test much for reasons I forget: just test it and it seems spot on. I'll return to it a bit more.
When I had some issues on my line (some time ago) I consistently got figures like
17 real upload,
17 visualware, << before it went bad
9 BT wholesale tester
6 BT retail tester << now defunct
Now my line is OK the differences are smaller.
17 real upload
14.6 BT wholesale
Moved (with trepidation turned relief) to BT Infinity 2 for upload speed. Happy BE user for several years.