There is a way that Virgin can operate into GLA without either rebasing an aircraft, or doing an empty sector to position the aircraft- it's called a W-pattern. For example, the aircraft could fly LHR-JFK-GLA-JFK-LHR. Crews, on the other hand, can be posotioned relatively cheaply- indeed, there are a good number of Virgin crews living in the Glasgow area and currently positioning to LHR.
However, I think this is unlikely- the transatlantic market out of GLA unfortunately seems too small to support a 747 load; in this case the US operators, having opted for smaller aircraft, have an advantage. I also wonder about the load a 747 could carry out of GLA- it is not exactly short-field ops, but it is shorter than MAN or LHR...