Since my update to m600 I am often finding that my upload speed is stuck at around 20 mbps - potentially during daytimes but haven’t extensively tested this - sometimes it does go to the full 40mbps but not often. This is to wired devices and directly at the router. (Tried all the basics, cables tight, rebooting etc.)
I have called previously and waited on hold only to be told there is no issue.
As you've requested Virgin's help, I'll leave this for them to investigate but would advise that this is likely a congestion issue.
Does the speed lift at around late evening and then nosedive again in the morning and daytime, if so this is typical across much of UK currently and is related to work from home use and increased video calling usage.