I second what 119.5 has to say. At this point, planning for the interview, you don't know how you're going to do in interview and, if you're lucky and good, training. You don't know for sure if you'll even like the job if you do get in! There are cases of people who get as far as validation training but either can't take the responsibility or don't take it seriously enough.
I went overseas after about 9 years with NATS, which I think is a pretty reasonable return of service, but I didn't even think about it till I'd had a few years in.
If you are successful in selection and training, put a lot of thought into what you expect of working abroad before taking the plunge. Any overseas ATS provider will expect a few years of valid service anyway, probably 5 years as a minimum. When or if you do get yourself a job overseas, it's not all a bed of roses! There can be hassles with work permits, going back to zero credit rating, animosity from colleagues potentially ("come over her, stealing our jobs", etc etc, and you might just miss the UK more than you expect. Personally, you'd have to drug me to get me back to Blighty, but after 14 months in Canada I have a credit rating and friendly local colleagues but it has taken all that time.
Going back to the main point; concentrate on getting the job offer from NATS first, then on getting through the college and then on validating at a unit. Build some experience and THEN think about working overseas.