You can currently still register for a standard Virgin Media PAYG account (ex-virgin.net, the ex-ntlworld PAYG services require an ntl line) at
http://allyours.virginmedia.com/html.../noncable.html - scroll down to the bottom.
[edit]Or, you could do what I do when I need internet access on the move, and just dial up to
http://emergencyinternet.com/ though my mobile.[/edit]