@bigblu,
It's entirely down to the the scheme you are on and availability. For whitetail you may well be given a choice at CTC depending on what's available, or at least I was. I opted for Arizona and Southampton. However, the MPL schemes may well be dictated by the airline.
In the case of the easyJet MPL I believe the vast majority have gone to Southampton and then New Zealand in the past, but who is to say that won't change in the future. A good example of this changing is the most recent Virgin MPL cohort whom will eventually head out off to NZ, or so one of them has told me. Whereas the Virgin MPL cadets very shortly taking up duties on the line and the two courses following them were assigned Arizona/
Hope this helps,