It relies on two broadband connection routes - your TiVo's (or V6's) outgoing connection from your home to Virgin and the connection from wherever the PC is (which may be also at home) to Virgin - if the PC is at home it'll still have to "talk" to the TiVo/V6 via Virgin's servers: If either (or both) ends are too slow at responding then the connection can fail. It could also be due to the Virgin servers sitting in between the two routing it all.
It's clearly not reliable, but that's not the same as the black-and-white statement of "it doesn't work" - for you it isn't, for me it is, and it did for the original poster back in September. Perhaps it'll be the other way around tomorrow and your will work but mine won't!? I would hope Virgin will look into this, but it may be that the issue is a combination of effects from the different kit in the comms path, which could include your TiVo, your PC, your broadband, Virgin's servers, etc. etc. They may just suggest the alternative option of using the phone app.
I haven't shut down the Chrome window I was using to connect to my V6 since I last posted and it's still connected fine to my V6s at home (even though I am at work on a less forgiving firewall and a much slower connection than my home broadband speed).
It may be the old TiVos are worse for connecting to, but I can't test that as I don't have one (but I did use this function successfully some years ago with TiVos, although things have probably changed since then).
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
It doesn't work for me and it never has. I have tried it on Firefox and Chrome, neither of them work despite numerous "try agains" and refreshes. I hear your reasoning about why it could fail but we are talking about the biggest telecom company in the UK for god's sake. This should just not be happening.
Step 3: Choose Device Management, then click “Manage My Sky Devices”, takes you to: http://manage-virgintvgo.virginmedia.com/skydevices After 22 seconds - "This site can’t be reached manage-virgintvgo.virginmedia.com took too long to respond." Did this 3 times and exactly 22 seconds each time!