I am currently having the exact same problem. Every time I use the Virgin TV Go on my PC it tells me I've reached my device limit and have to replace an existing device but I'm locked out from doing that until Thursday 1 August.
I also have the browser setting set to not clear anything for the Virgin TV Go site but that doesn't seem to make a difference and logs me out almost every time and then when I log back in I have to re-register the same device every time. This issue only seems to have started when you updated your site quite a while ago (like months and months ago). Having the site set as an exception when clearing cookies etc worked perfectly before this on the old version of the site. The site would stay logged in and I'd never be asked to re-register the device. Since the update however, it logs me out almost every time and asks me to re-register the device. And note I said almost every time, as occasionally it does keep me logged in for some random reason. There doesn't seem to be any rhyme or reason as to why. Anyway, before it would just make me log back in and ask me to 'Register a new device' every time I tried to watch anything after logging in. It would never ask me to replace an existing device but simply to register a new device. I did this every time I logged in, so like a hundred times, and it was annoying but worked every time. However this has changed the last week or so and now it asks me to 'Replace' an existing device. I did this for a number of days but now says I am no longer allowed to do that and has locked me out until August.