I recommending contacting Virgin and querying the bill.
They will tell you straight away what the limit is, if there is one. As far as I was aware, they have the Acceptable Use policy, which usually comes into force when you are using far too much resources that it could degrade the service received by others in the near-by area. If you are using a substancial amount of the resources, they are entitled to restrict or even in some cases cut off your service until your usage subsides - that is my general understanding of the Acceptable Use policies.
Being on Virgin Cable myself, I am subject to the Peak Time speed policy, but as for the Virgin service as a whole, I have understood it to be cap-free
Anyways, like I said, contact Virgin and ask them. They are the best ones to tell you.