adfly
The large white/grey aircraft you are talking about is an Airbus A318-112(CJ) Elite (LX-GJC) of Global Jet Luxembourg. It has been there since the middle of the week and departs on Sunday. It has chosen to park here because Southampton is just a little cheaper to park an A318 than LHR!!
You may have noticed it has been reversed into stands 13/14, thats because the tail is too tall if parked nose-in as normal and would be over the height restriction for the stands 6-13 and they don't want it taking up stands 1-5 for the period it is in SOU.