Looks like I'm on a fixed rate 1Mbps connection though I pay for up-to-8Mbps

loaner_2

Your ISP should be able to tell you everything, they should know whether you are on adslmax or not, and what your profile is. They are also responsible for liaising with BT for problems, BT probably won't discuss it with you, because the service is via a 3rd party.

The BRAS profiling is automatic.

ADSLmax is typically 448 upload speed unless your line is really bad, the profile theory is speculation until you ascertain which service you are on, it could be profile issue, but more likely you aren't on upto 8.

macman

I agree with previous posters, you need to isolate any potential problem with your own wiring by retesting from the test socket under the master socket faceplate. This is the first thing BT would ask you to do anyway in the case of a reported line fault.
The other thing you can do is to test your ADSL filters by swapping each one with a known good one and retesting each time. I'm asuming that you do have a filter on every active extension, including any Sky boxes?