Hi there,
Although they used to, NATS don't actually look specifically for ATCAs anymore and so you'd only really find ATCA vacancies at some airports not controlled by NATS, which rules out quite a few. However, NATS do still allocate controllers to be ATCAs from the training courses - they just don't train controllers to be ATCAs for their whole career.
Best thing to do would just be apply for NATS - after all, the requirements are generally the same to be either an assistant -or- a full controller.
Hope that helps
