If you're on Mix TV or above and have broadband with Virgin, you can get the V6 box for £20 upfront and no monthly increase. Also if you speak to the Retentions team and re-contract your services, you might get it free. Most of the customers who Virgin have contacted offered the V6 for free are those with legacy boxes that are soon to be withdrawn.
Considering that there are still boxes on the network that are >15 years old, you could be waiting a long time for them to contact you about your TiVo box, which runs on the same platform as the V6 so there are no technical reasons to withdraw it. In addition, customers taking TV without broadband can only get the TiVo box.