The home hub has to try all possible combinations of connections as it has to deal with everything from ADSL upward, as its not possible to fix the connection profile, so a bit of noise on a slow connection, can really upset it.
You may be better with a third party modem/router costing £35, which does allow you to specify the connection profile. Its worth a try, as its quite popular,
This one allows you to do that https://www.tp-link.com/uk/home-networking/dsl-modem-router/td-w9970/
There is a software emulator which you can play with.
You would be better on ADSL, as its shown as available.
It comes from the exchange, so the fact that your main cabinet is linked to a fibre cabinet, has no bearing on that.
The problem is that it seems to be impossible to order an ADSL connection, if there is FTTC available, even though the connection may be worse.
It should be possible to override the order system, and allow an ADSL regrade to take place, otherwise there is nothing you can do.
You are well below the minimum USO speed, so BT should off a 4G mobile connection, provided you are in a 4G coverage area.
How would the OP be better off on ADSL with an estimated speed of up to 1Mb?
Being below the minimum USO speed does not automatically mean the offer of a 4G connection unless
"You’re not due to get access to one (USO) through a publicly funded scheme within the next 12 months".
The OP can check HERE if this is the case or not and what options are available.
We are now less than 3 years from the stop sell date for ADSL by Openreach so I doubt ISP's have overides in there order systems now.
So... its likely that the _actual_ noise on the line (not whats measured) is disturbing the negotiation.
Therefore one conclusion is that the designer of the vdsl negotiation algorithm didnt expect to cope with long runs of copper outside of a test lab ! (Surprise!) AND the test dept didnt know how to do 'real world' tests (Surprise!)
I think my valley (like many others I am sure) is the victim of an 'inky' in an office deciding that was where the cabinet was going to go, and (as usual) any engineering input as to whether it would be 'in spec' for all customers was ignored.
Thank you for the USO info as I was unaware of the legal support for a 10Mb service.
I think you'll find that 'inky' in the office was planning just a PSTN service and had no inkling if what an internet connection was.
I think that @Keith_Beddoe suggestion for a cheap third-party modem/router is likely your best option as it doesn't have to do so much re-negotiation as a SmartHub. The TP-Link recommended by Keith is well suited to BT Fibre installations.
This doesn't really answer your query but probably provides the best solution.