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.
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.