Also B744 gates are limited, especially if there is a lot of 340's around, and also with all the construction that is going on. Another reason why you possibly waited so long, is that aircraft that are being towed onto a gate for departure also take priority over other traffic.Virgin might also have requested the crew to hold for that stand as the aircraft could have been departing within a couple of hours and wanted to avoid all the problems that an off stand departure can cause.
HZ123 you are correct as the BAA gives a 10-15% discount on charges for flights that go off stand.
All the best
SW