This post may explain the reason, as you are sharing the upload backhaul of 1.2Gb with up to 32 other users, so if they are doing a lot of uploading, then that will reduce your upload speed.
https://community.bt.com/t5/BT-Fibre-broadband/Low-upload-speed/m-p/2251355#M340266
I do not think that BT guarantee upload speeds.
Testing 2 of my neighbours today and they all consistently get 110-120Mbps upload. On the phone to BT now and they've confirmed a fault and sending an engineer. Fingers crossed it'll be resolved soon.
Quick update. As if by magic, at the weekend the upload jumped to where it should of been.
Hi @Dialupp and thanks for letting us know your upload has improved.
Out of interest can you just confirm how long is it since you were activated until the upload changed?
Cheers
David
It was about 3 months if I remember correctly.