Presumably you will want a PE flight. Shorter is usually cheapest. (London to New York, Boston, etc.) I don't know if including a Belfast or Dublin flight can make the itinerary cheaper than just flying from London. (and positioning separately)

In any event, it's much better to just go somewhere that appeals to you on dates that work for you. Otherwise it seems like a lot of hassle just to save some Virgin points on a 2-4-1 booking.