Failing the course would not do you any favours. These days you are streamed into aerodrome, approach or area (I believe in the past you would have been trained in multiple disciplines). To do any of the three disciplines you still need to pass the basic course to the same standard. As has been mentioned, you do get the chance to state your preference of discipline, but it is not guaranteed if it does not fit with the business requirements. If your choice is to be an area controller, you will probably get your wish as the majority of trainees are destined for area. Aerodrome usually has fewer places than trainees who want to do it, so some will inevitably be disappointed. And the company will of course post you to where they need you. This is the deal that you sign up to right from the start, which NATS make very clear.