Yes type in the www.virginmedia.com/v6upgrade into you top search bar not via google or any other search engine and it all works straight away, have succeeded in ordering mine this morning,. Hope this makes sense. Good luck.
Not sure if you've got this sorted or not but if you use Google to search then you won't find the address. If you type virginmedia.com/v6upgrade into the address bar direct then search, it'll take you direct to the site. Alternatively, the link to the site is here: https://keepup.virginmedia.com/v6upgrade
I too received a letter advising that I needed to upgrade my existing system to the new Virgin TV V6 box "completely free of charge", however when I tried using this link I received an on screen message informing me that their system doesn't recognise my details (Name, Address & Postcode) which I find strange as it's exactly the same details they used to send me the upgrade letter!!
Also they seem to know exactly who I am and where I live when it's time for the monthly bill to be paid!
Received a letter saying free upgrade but refuses to acknowledge that i am a customer. Evidently they "can’t find your details in our system. Please double check your address and try again. Only eligible customers for this offer are able to order the upgrade online."