This post only focusses on the theory element ...
Zen don't want to do anything due to being above 55Mbit, which is odd as that is the Impacted rate but I'm fairly sure my line has always been considered Clean.
This classification is confusing, as it really doesn't have anything to do with the actual state of your line, and more to do with the trust in the state of your line. From then on, it really only relates to what guarantees an ISP is willing to honour.
The way in which the state can be trusted is by getting the installation done by an engineer. At which point the ISP (and Openreach) will trust that the line is clean, and willing to send engineers out to look at the line when it gets to the handback speed of the clean line.
The distinction came into being when Openreach allowed self-install to take place. Lines that are self-installed are not trusted to be clean; the quid-pro-quo for a cheap installation would be that the ISP (and Openreach) would then only honour the "impacted" speed, and send out engineers when the lower threshold is hit. Note that the line might actually be clean, but no-one knows. And because the subscriber chose not to pay for the engineer installation, there is no basis to trust whether it is clean.
Move on a couple of years, and the position
now is a little muddier. Some ISPs quote the clean speeds (eg Plusnet), while some quote the impacted speeds (eg TalkTalk), while some quote the entire range of clean and impacted (eg AAISP). Nowadays, all that really matters is what range the ISP chose to use when "guaranteeing" your speed were first installed.
Of course, migrations muddy this picture even further. An engineer installation with Plusnet, migrated to TalkTalk, does not necessarily take the "trust" with it. And, while an engineer visit might change a line from actually-impacted to actually-clean, it doesn't necessarily change the ISP's trust level for future faults.
However, I don't know what Zen does with either installation or with the personalised speed estimates on signing up. But it likely fits into the above picture well.
From the BTW page it sounds like Impacted is only if the physical line has issues, which mine does not.
The distinction arose at the time self-install was introduced. The *real* problem that BT wanted to avoid was allowing a cheap self-install happen, but then being called out because of bad house wiring. My starting point is to think that "impacted" = bad wiring inside the home.
Does a climb in crosstalk suddenly re-classify a line as Impacted?
No. The range in speed (on both clean and impacted) is meant to cover the variation caused by crosstalk.
Each range, incidentally, is meant to reflect the 20th and 80th percentiles - so 20% are faster than the top, and 20% are slower than the bottom.
The handback speed is meant to represent the bottom 10%, which represents the cases where BT are willing to send an engineer (in cases when the only evidence is the slow speed; they'll send engineers where line tests fail irrespective of speed).