I get the full upload speed on Plusnet 40/20; a bit above 18.5Mbps actual throughput.
Upload speeds seem to vary more than download with different speedtesters. I have found the ThinkBroadband speedtester to be the most accurate; it gives the highest results which tally with the best results I get to good real sites.
If you are using the openreach modem and have it unlocked, turning off QoS adds around 1 to 1.5Mbps to upload.
Wrong MTUs can cause poor upload speeds. These days most systems seem capable of getting that right automatically, but if you have got some older equipment or equipment (computer or router) you have manually changed MTU it could be the cause.
Recently moved from BT Infinity 2 to PlusNet. Very happy so far.