From the DSLStats snapshots provided which I'am assuming is your line from the DS synch speed listed on the graphs:
From your original post line stats, DS attenuation indicated 31.1 dB, certainly quite a long line.
The DSLStats snapshots provided would indicate that this is correct and your current DS synch of 27.96 Mbps is probably as good as it gets on your line, D2 bit loading is very low, D2 tone range on QLN graph is quite noisy.
Your US synch speed is very poor for the contracted package of 40/10 at 1.2 Mbps, U1 band bit loading is very low, U1 tone range on QLN graph is noisy.
My opinion, a long line showing the expected VDSL-2 characteristic of performance droop at higher frequencies, as per any other VDSL line, very likely being effected by cross talk from other users on the same fibre cabinet.
Probably not a lot you would be able to do to improve the line performance and from my 10 year patronage with Plusnet I would be very surprised if you could get them interested.
As for your neighbours TT VDSL-2 line, ideally would need some stats similar to the DSLStats provided for your line to evaluate.
All I can suggest is to temporarily replace your neighbours modem/router with a suitable Broadcom chip device and perhaps run a RPi on this with DSLStats, I did exactly this for my neighbour who is also with TT and was having line issues, set up with VNC the stats can be monitored remotely.
Still think that your neighbours line is shorter than yours as his DS attenuation indicates 25.4 dB, this being correct, in theory, should perform better than yours on a similar modem, however, as other forum members have said, every line is unique with respect to DSL performance.