Practicalities of the new DOCSIS 3.1 standard.
It will allow more efficient use of the spectrum on the cable networks, so without any upgrades it's possible for Virgin Media to offer higher speeds.
With upgrades to the cable networks to increase their bandwidth, and by this I mean their RF bandwidth, it's perfectly feasible for VM to be offering speeds matching the 1Gb downstream, 100Mb upstream service BT are trialling to some of their footprint by next year if they so choose. The upgrades are ongoing and anywhere that has had them can, simply by changing the equipment at the customer end and some line card and software upgrades at the VM end, deliver the 1Gb/100Mb service I mentioned.
Even without going to the new standard VM should be knocking on the door of BT's final G.fast speeds, 500/50, by 2017 if they're minded to.
Virgin Media Ireland have just released a 360Mb/36Mb service over DOCSIS 3.0, not the newest standard. Comhem in Sweden offer a 500Mb/50Mb service over DOCSIS 3.0, too. VM UK could certainly match this next year, however whether there will be the competitive need to is debatable. For now Ireland and Sweden have way more FTTP delivered and in prospect than we do, and G.fast will still be in pilot phase in 2017.