On Sky all of the ITV (and BBC) regions have individual SD channels. When you program the box the channel(s) corresponding to your location are simply assigned to the number 103 (and 101,102 etc).
You can watch the other channels if you find them in the EPG in the 900s or something.
However with HD the individual channels for each region don’t exist. There is just one HD channel for the whole country. BBC choose to display a holding message when local programming is on saying to view it on the SD channel. I assume ITV simply choose Granada as the “national channel” to broadcast on the HD channel.
I assume its the same on Virgin.