Hi,
I am an EX Virgin Installation Engineer perhaps i can help. Virgin has its own dedicated exchanges and telephone network, hence the need for a seperate phone socket from Virgin Media. If you are looking to leave Virgin media BT will have to arrange for an engineer to reconnect a BT line at the Exchange.
BT will also make sure the line is working up until your master socket and doesnt take responsibility for extension sockets. This you need to consider if your Virgin engineer moved your extensions into the Virgin media socket.
A good engineer should have left your BT Socket intact (BT's Property) and placed the new Virgin media socket next to it and simply moved the extensions over. This can be easily reversed with a bit of knowledge.
Hope this answers a few questions.