Are you using a wired Ethernet connection to the VM hub or a wireless connection? Wireless is easily disrupted by a number of external factors, so a wired test is the best way to get a true picture of what speed your line is able to achieve. It also helps to pinpoint if the issue is with the wireless or your line.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out. My setup: V6 TV box, Vivid 200 Optical fibre with Hub 3 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.