Hi there, I am in the same boat. I have my own router and I wanted the modem only mode which Virgin reassured me it is possible with the hub they provide. To my surprise when I loged in the settings of the hub 5x there was no option for the modem only. There is a way to navigate to the modem only setting page by changing the url in your browser, however when you activate it the router goes into stall mode and you have to reset it.
My solution for the moment was to keep the DHCP server on in the virgin hub, connect the ethernet cable to port 4 (10GBPS port), connect the other end to my router WAN port and then set the WAN setting from PPoE to DHCP. That way my router got a unique IP address for the WAN from the DHCP server of the virgin hub but kept all the static IPs I have set up around my house on a separate subnet.
Then I deactivated all the firewalls and wireless networks of the virgin hub.
Now I am keep checking every now and then the settings page of the virgin media hub for when the modem option only option appears. I spoke with customer support and they aim to have the feature released in the next 3 months so maybe you are lucky and it will be available when you get connected.
If all fails remember you have 14 days from the day they connect you to change your mind and cancel the contract.
Hope all these help.