What HP said, basically. The BT database is often ropy. I still remember back to when I moved in here and tried to get ADSL in summer 2004. The BT checked said I could only get 512k. I tried ordering a 1Mb service anyway and it was rejected, so I had to order 512k.
MrSaffron from ADSLGuide (as it was then) kindly used a contact he had at BT to get the checker updated, so I was able to upgrade to the dizzy heights of 1Mb!
I'm now syncing at ~10.5Mb with Be - all on a line that BT once said would only support 512k
Have you tried putting your current line stats into
here and see what it comes up with?