The cap may be the IP PROFILE which I posted a link to an explanation of.
You can find what this cap is by visiting
www.speedtester.bt.com
This should improve if you line holds the current 2324 Kbps for long enough and then see you getting figures of 2 Mbps from speedtesters.
IMPORTANT
Speedtesters also vary in how they work, ours takes a simple average result for the time taken to download the data, What you will see is this
1. Small burst of download
2. Small burst of upload
3. Measured download, amount calculated from 1
4. Same as 3 but for upload
If you use the flash based tester you also get graphs which you can compare with the NetBarrier
The ookla (speedtest.net and many others) based testers, take a number of 30 samples, discard a number of the slowest and highest, and then pick the highest sample from the remaining. Thus this will tend to give a higher result than a pure average.