Short answer. You cant. The cable Virgin use has to be specially made as the cable system re-uses frequencies that are used "outside" the cable for terrestrial TV & DAB radio. Any ingress of these signals can mess up the whole of the local network. Lengthening the cable can also affect your signal levels & hence performance of services. For this reason any DIY alterations on Virgins network cables are against your terms & conditions of service.
You have 2 choices:
1) Get a long HDMI cable & move the TV. leaving the Virgin box where it is.
2) Pay Virgin £99 to come & alter their cabling.
Cable customer since 1993. Services: FH TV, Sky Sports & Movies (2xV6), Talk Anytime Phone, VIVID 350, Virgin PAYG Mobile