Mobile Broadband dongles use the mobile phone networks and performance varies dramatically depending on levels of traffic on the network. I have a dongle on "3" and at about 7 o'clock in the morning can get a download speed of about 1Mbps. In the evenings, I have had it fall to about 280kbps. These are "speedtest.net" results - in practice the connection can seem to be considerably slower than that.
There is absolutely no way in which you would get a performance comparable to your 10Mbps Virgin Cable with a Mobile Broadband connection.