As you mentioned, the UK ANFP governs FTTC here, and only allows BT to use a maximum frequency of 7.05MHz from the cabinets and 3MHz from the exchange, which is why we cannot get VDSL2 from exchange lines.
This means that we also have to follow a truncated 997 band-plan up to 7.05MHz only, which is what Profile 8c is all about, while profiles 8a, 8b and 8d extend to 8.5MHz, and profile 12a and 12b extend to 12MHz. Interestingly, the portion of band-plan 997 above 7.05MHz on all of those profiles is for UPstream data only.
This document by Ericsson
has an alternative graph of VDSL2 vs Distance (page marked 43). The graph is titled "VDSL2 bit rates of different profiles. Real measurement in the presence of crosstalk from 20 VDSL2 systems." It includes profiles 8d, 12a, 17a, but all look to be most bound by the upstream service.
The crosstalk obviously has an effect there, because that shows considerably lower speeds. It shows 40Mbps for 200m or so, and is probably nearer to the kind of estimates that BT's checker is making.
I found another Openreach document
that includes a 2009 primer for VDSL2, and includes infomation about the ANFP and band plans.
But that document, page 27 on, has an interesting piece about what Openreach has to do to make sure VDSL2 doesn't interfere with concurrent ADSL2+ signals fed from the exchange. Basically, they have to apply a "power shaping mask", which reduces the power of VDSL2 signal frequencies in the ADSL2+ frequency band; it reduces the signal in the cabinet down so it has the same level as would be expected for ADSL2+ signals that have arrived over the E-side wiring.
That means that, as the cabinet gets further from the exchange, the power mask is adjusted to reduce the power of the VDSL2+ signal more. But conversely, the cutoff frequency at the top-end of the mask can reduce as there is less expectation of the higher-frequency signals reaching so far.
This "power shaping" suggests that the speed of FTTC's VDSL2 isn't *purely* affected by the length of the D-side wiring (like we have always believed). The speed is also going to be affected by the power-shaping applied, and that depends on the length of the E-side wiring.
That part is all new to me... but from the references to ITU specs, isn't new to Telcos.