HP,
You'll be on one of VS's 'Gatwick fleet'. This is the bucket and spade brigade, ex-NZ aircraft.
The config is as under_exposed describes. The Y seats are the older variety, but actually better than the newer models on VS A340's (slightly wider and slightly more legroom). Premium on the upper deck is good value due to config , seat, and crew availability. Upper Class uses old Recaro seats (Not VS product) and should be avoided!
VS are currently selling exit row seats, best to give their Reservations line a call and see how this works.
Have a great trip.