Looks like Virgin customers will be using the new combined EE network
from the summer.(Look in the 'And there's more to come' section.)
"
We’ve always used the T-Mobile network, which covers over 99% of the UK population to provide our service. But from this summer, we’ll use the Everything Everywhere network (the company formed by Orange and T-Mobile). So in some areas where the coverage would otherwise dip, you’ll automatically be able to use either the T-Mobile or the Orange network at the same great Virgin Mobile rates."