Virgin have a lot of over subscribed UBR's (the network areas) and that affects speeds, but they are migrating the 10meg and 20meg users will be ported over to the DOCSIS 3 network but will be running DOCSIS 2.
I think they will need a new ambit 256 modem to get the benefit.
Go to
www.virginmedia.com/swapmodem and you can order an Ambit
The DOCSIS3 is primarily for 50meg users but they will use the DOCSIS 3 network as opposed to DOCSIS 2.
This theoretically should open up more bandwidth and increase end user speeds.