Honestly, too many people suggest modem mode as a magical fix for all sorts of issues. Modem mode actually has fairly limited use cases, it's for power users who would rather offload some Superhub functionality like DHCP or WiFi over to a device of their choosing. Offloading WiFi isn't a bad call, considering Virgin Media's routers have the stupid decision to constantly reboot if the connection goes down, nuking your WiFi. I digress.
Anything to do with QoS is basically pointless, Virgin Media will completely ignore any markings you throw at it and apply their own policies to protect their network and services (no funny ideas about marking all your traffic as EF). So all you'd do is get a router to apply a QoS policy which gets completely ignored when the traffic reaches the SH or headend. If you're having congestion issues on your LAN before it even gets to the router then you have other problems to deal with.
The way I solved bufferbloat was by switching to FTTP, which despite a technically slightly slower connection speed resulted in a massive increase in performance. Otherwise it'll be a long wait for VM to implement low latency DOCSIS which will no doubt happen eventually but they won't exactly be in a rush.
