There is a difference in the Ireland and UK Virgin Media Hubs in that the Ireland one is IPv6 and uses DS-Lite which won’t work for port forwarding.
There is no way in the modem set-up to change this so one must phone Virgin support and ask them to change the configuration of the HUB to IPv4. I was quite surprised that the support person I got said “no problem, I’ll send an email now and the router should reboot in the next 30 minutes and IPv4 will be configured”. It took about 10 minutes.
UPnP was enabled by default so Roon automatically configured the port forwarding.
This is what I found out about DS Lite for my Virgin router
The reason why the port forwarding option is missing is because with DS-Lite, your IPv4 address is not actually routed to you. It is coming from a CGNAT device in the Virgin Media network, meaning it is shared across multiple VM customers, not just you. This makes port forwarding impossible, because the IPv4 address you have isn’t directly on your router, instead you go through a local IPv4 subnet which then is translated to an external IPv4 after going through their network
Might be worth contacting your ISP though as I actually had so little trouble I bought a lottery ticket believing myself bewitched with good luck.