I too am experiencing this issue.
It has happened in the past every so often and switching the Ethernet port on the Hub 4 that my router was plugged into used to solve the issue. (I run the hub in Modem mode using a UniFi Gateway).
We had an outage in our area about a week ago and since then the issue is more prevalent and does not sort itself with either a reboot or different ethernet ports.
If I unplug everything and leave the Hub 4 to cool for a while sometimes when switching back on I get the full gigabit speeds for a while then it reverts back to a 100mbps connection.
Not sure if it is the router overheating or a firmware upgrade error or something to do with the outage the other week but very frustrating.
I have tried swapping out ethernet cables in case of faulty cable but makes no difference and they are all cat 6a cables which previously gave 900+ mbps.
Nothing changed in my network and it seems odd that I can get the gigabit speeds if only briefly
Suggests to me that the Hub when on modem mode is setting all ports to 100mbps rather than 1000mbps. Not sure if that is down to overheating, faulty hardware, a software bug or error or something else.
Notified virgin today and they have told me I need to call back after 24 hours again.
With the amount of comments about this issue it seems to be something that virgin should be aware of and hold their hands up to by now.