Re: You can't do it anyway.
If the Virgin provided router you're talking about is the SuperHub and it's in modem mode, and you use a decent router of your own, the option is there.
I didn't notice the problem for the obvious reason. I just wish my supposedly 'up to 152Mbs' connection showed me a bit more oomph than the 40-50Mbs I usually get1 - consistently from speed test sites.
When the Virgin Media guy came out to do the installation and pointed out I wouldn't need my existing router, I said I'd be putting the SuperHub straight into modem mode so I would need my router. His reply was a small laugh and the comment "Each to their own" as if what I said was crazy talk.
1. To be fair to them, though, looking at the Superhub's status, I see it says the maximum speed is something around 160Mbs, so maybe I should replace the old/cheap cable running from it to my router before complaining. Just in case it's not up to the job.