I don't think the 1.43Mbps is anything to do with the Current line speed
. It is something else strange in parts of Plusnet's network. I often have the same effect, but it doesn't seem to affect live working.
I would expect a higher than 5.7Mpps setting if your IP Profile is 6.12Mbps. More like 6.1.
Limiting your output speed from an ISP's servers is actually quite common, if not ubiquitous, though few talk about it. If I'm not mistaken both Zen and AAISP mention it for example. The reason is that your stream is then buffered within the ISP's network rather than suffering huge packet loss when it hits the MSAN/DSLAM at full speed.