Just stripped back the white outer sheath and the cores aren't twisted. Is this correct? Thanks and apologies for more questions. I have previously trimmed back cable on a master socket and used an idc tool to remake the connections and saw the attenuation drop a little! 🙂
Firstly, not your cable to be stripping back, secondly how are you totally sure it is not twisted? Cabling is not always obviously twisted.
The black box is likely just a BT approved joint enclosure such as this snapped and closed:
https://www.picclickimg.com/00/s/OTAwWDE2MDA=/z/xdMA...
It is standard to use internal vs external cabling, one white and one black. Drop cable would not typically be used internally, it will often run outside, drill through the wall and straight into the master. If there's internal wiring, it should be white as yours is.
As a reference I have an attenuation of 13.9 and my green FTTC cab is less than 50m away. It all depends how the cabling is routed.
Third point, you state you are upgrading to 80/20, although based on your signature "Predicted speed 54.7Mbps down/15.7Mbps up. Actual speed 42Mbps down/4.5Mbps up" indicates you already had an 80/20 package on BT?
Note, DLM is the same on Sky and BT, so in theory your speeds could end up the same as with BT. Based on the noise margin of 13.1 in your signature, you know it is extremely unlikely you will get 80 down right? I would suggest 50 to 60 is likely, depending how the line stabilises.