Just to add to the above comments, if you do apply and you are still given the chance to put your preference in, I would suggest that if you want to stay in London, put Scotland as your first choice and you might just get London.
On my course there was a 60/40 split between area and airports, the split actually worked in favour of everyone's choices and you would think that the company would welcome the easy decision, abilities aside. Alas they decided to send people who wanted area to aoerodrome and vice versa. I say abilities aside, as when asked about the decision we were told that it wasn't about ability, but where they thought we would "fit in"
My opinion, don't trust NATS management, as has been suggested before, some people in charge of these things take great satisfaction from sending people to the opposite end of the country than their preference. Staff morale is not a priority of NATS HR.
As an aside I'm quite happy with my posting, so I'm not a bitter contributer, just posting what I experienced.