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 just with the wireless or your line.
You can call 0800 561 0061 to check for known issues in your area.
If the speeds are low with a wired connection and there are no known faults in your area then it could be an issue with your line and some hubs stats might help diagnose the issue. Go to http://192.168.0.1/ (or http://192.168.100.1/ if you are in modem mode) to get the VM hub GUI. Don't login (unless you have a Hub 3 and this is your first visit to the hub GUI, which does require you to login) but instead click on Router Status which is either a button in the top right of the screen or a text link at the bottom of the screen depending on which hub you have. Click on the Downstream, Upstream and Network Log sections and copy & paste the figures from those screens and post them here, don't worry about the formatting or you can post screenshots if that is easier. Be careful to edit out or obscure any reference to your WAN IP or MAC address in the log to protect your privacy.
The community can have a look for you and see if anything in the figures looks wrong and advise accordingly. Or you can wait for the VM staff to pick up this thread in a few days.
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.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.