If you look at the community this has been mentioned a number of times over the last few months.
It is nothing to do with Virgin Media as you will see exactly the same discussions using other ISP's such as BT.
It is a phishing scam that is triggered when accessing a particular website.
If you are brave, or foolish, enough to follow it through the end screen will ask you for your card details to pay £1 for the iphone you've won, with of course everyone being a winner.
I had a cyber security professional look at it and he assures me it is not related to malware on the device but is a rogue link embedded in a corrupted pixel within the website that you were in when it was triggered.