I thought I would give an update to anyone interested in this thread. An Openreach senior manager personally monitored this issue and kept me informed by email and the BT mod team kept in touch also. Openreach monitored the cabinet after my first email request to them and found the capacity issue at the cabinet at peak times and decided to make upgrades to the cabinet and add some extra hardware at the exchange.
The work was completed at the end of September. They installed some hardware at the exchange (I'm not sure what this was though) and added an extra fibre link to the cabinet DSLAM a day later. I'm unaware of how much bandwidth was added but I can happily say that it's Friday night, peak time and I'm getting full speeds and zero packet loss. I'm only on the 50mb package so unable to determine if the Fibre2 package would give close to full speeds at peak times, but the speeds are solid throughout the day/night. Playing Stadia shows minimal to zero packet loss.
My router stats currently show (TP-Link Router) (I'm on the Fibre1 50mb Package):
Current Rate (kbps) 9997 54999
Max Rate (kbps) 22118 86979
SNR Margin (dB) 13.4 9.8
Line Attenuation (dB) 1.4 11.8
Here are some speed tests (The only limiting factor now seems to be the Fibre1 package itself):
So all good in the end and I hope this thread will help both customers and the mod team in the future and also save BT some money on extra engineer visits. Hopefully in the future there will be some more careful monitoring of the cabinets and faster upgrade times, as well as closer communication between ISP's and Openreach about the cabinet bandwidth capacity at peak times. Thank you to the mod team also for keeping track of everything and making sure the issue was fixed.
Congratulations on your detective work and persistence! It's interesting that the network is so opaque and that these fundamental infrastructure shortcomings need specific monitoring and aren't automatically flagged. How efficient can their investment in capacity be if it takes someone with your acumen to alert them. Well done.