Yes, NATS CTC is the only place you can do the area course, but they don't decide what route you go down until you are some way into your course (at the end of the basic course I think). So if they decide you're best suited for area control then they would send you to CTC after doing your basic course even if you did the basic at Global. They are sending a number of people to Jerez in Spain for the basic course and then following that they have to come back to the UK to CTC or Global to do the following courses, so I don't imagine they have any qualms about changing training locations after the basic course.