The 1Gbps speed to the hgub4 is being hit, it's the speed to the devices that is slow.
Then to be blunt, it sounds like your device(s) are the issue.
The Hub is receiving 1gig (higher actually).
The Hub is more than powerful enough to sustain over 900Mb/s over Ethernet.
That leaves your laptop/pc.
Having a gigabit lan card isn't enough. Older CPU's just can't handle 1 gig throughput.
Run a Linux Live OS and run a speed test to rule out the OS.
Virgin are quite clear during sign-up of the 1 gig packages that they guarantee your speed to the hub.
Your devices are your responsibility. Quite right too.
Not sure why you've opened a complaint.
Seeing as you bought cat8 cables, I have some magic beans you may wish to purchase.
Edited by j0hn83 (Wed 13-Jan-21 00:07:24)