Surely if I'm getting a good speed using the speedtest login it bypasses the ISP therefore they are the issue?, or am I missing something.
I've just been reading through your thread with interest as I've recently had a similar problem to you. I am also on a 20CN exchange, max line speed possible is 8Mb (my line syncs around 7200Kbps).
Up till the end of October I was using Freeola (an Entanet reseller) my download speed up until September were always around 4.5 - 6 Mbs depending on the time of day. From September my speeds whilst using the BTwholesale speed tests ranged from 1.5Mbs to 0.01Mbs (No that's not a typing error). I finally did the level three test and I got a download speed of 6.1Mbs straight after the 0.01Mbps result.
I was on a business tariff with an 0.8Mbs upload, so allegedly had a superior level of throughput.
Anyway, I moved on November 1st to Zen, immediately ran the BTwholesale speedtest and my downloads were back to 6Mbps again
and since, speeds are once again fluctuating between 5 & 6.
So, after a long explanation, Yes your ISP is the issue.
Request a MAC, I know I did and am pleased I've moved away.