I've had the same kind of issue. i can't use the Virgin app over wifi. If i want touse that app i have to disconnect my mobile from my home wifi. Of course after using the app (which would only be a quick thing) I then have to reconnect with my home wifi. Such a stupid app fault!
I am still having this problem, although I think a virgin person on here said i need to have my account reset by calling 789 (havent actually got round to doing this yet but will be interested if it works for you!)