With the choices of discipline, you get to choose on a form if you would like Aerodrome or Approach. If you don't fill in this form, then you go to area...If you do fill in this form, it is still highly likely you will go to area
Luckily, I filled in this form, and got what I asked for!
As for the numbers, on my course there were at least twice as many people requesting Aerodrome or Approach, than there were numbers available...So if you choose an option on the form, don't bet on getting it. I would still think in the back of your mind, "I'm going to Swanwick"...If you get what you wanted, its a bonus!
DF