Fares: Those are more like it. On checking your dates I came up with £529 on the Virgin website for economy. However, returning on the Friday, it still shows £1,335 for economy. Maybe that is already a busy flight.
Out of interest pwalhx, where did you get those fares from. United's web site came up with £687 economy for your 5th/11th dates and still £1,236 for the 5th/9th despite the flight being wide open in all classes.