I think it's just wishful thinking that you've got a contractual right to the highest sync speed free of whatever things restricting it that you don't like.
[rant]
I think BT have subtly changed the goalposts over time.
In ADSL and ADSL2+ days, BT/Openreach set their thresholds WRT engineer involvement, but ISPs had some flexibility over the line relating to DLM.
In original FTTC days, Openreach worked to similar thresholds WRT engineer involvement, and gave ISPs no flexibility relating to DLM. The attitude to the former says that BT are willing to investigate and fix physical faults, but they are forcing (some) ISPs to ignore unprovable faults. The attitude to the latter says that BT consider DLM to be vitally important, *and* that it is perfect in the decisions it makes.
[As a software engineer, I can say this: it is rather hard to make software perfect. If you can't make it perfect, then make it self-correcting]
The older variants of DLM weren't perfect. But they did relent, slowly. So, even if DLM made a bad choice sometimes, and got involved when it didn't need to, it would eventually remove itself. Thankfully.
The ability to self-correct meant that DLM's failure to be perfect was not a fatal flaw.
In current FTTC design, Openreach continue to make (some) ISPs ignore those unprovable physical faults. But as a consequence, they are also forced to ignore faults that DLM has latched on to and not let go.
The current variant of DLM seems to make bad choices - including being far too prone to using banding. It compounds this problem by having removed (or, at least, reduced) the self-correction. Banding, sometimes, is sticky.
To me, that looks like DLM is fatally flawed software that has inherited an ego problem from its designers: it cannot cope with its own flaws. DLM believes itself to make perfect decisions, and to have no need for a human nudge that says "Are you sure?" DLM is now the all-powerful arbiter that, sometimes, stops you getting the speed you ought to, but cannot see it.
And, because ISPs like TalkTalk aren't willing to get an engineer involved, you've lost any manual backstop.
If the Openreach platform had a button labelled "Please ask DLM to try relenting", TalkTalk would hardly refuse to press it.
