There are several posts on this very topic and, as far a I know, none has yet been answered by a Virgin Forum moderator despite the query being escalated by a kindly VIP.
I have simply want to review my latest charges because my balance has dropped. Despite the 'sign in' page finally appearing this morning (after days of 'Oops, the website is being maintained), it did not proceed and now my browser says it may be busy, try later.
It's an upheaval to move provider which is a disadvantage for customers. If it had been easy, I would have probably left after the SIM change debacle.