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.
Are you seeing slow speeds all day or only at peak times (typically 4pm-11pm weekdays and weekends)? Some areas are suffering with over utilisation, which results in very slow speeds at peak times but full speed at off peak times.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong If my answer solves your problem please mark it as helpful as it may help others My setup: Vivid 200 Optical fibre with Superhub 2 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.