At the moment, NATS just needs suitable applicants. They aren't going to offer a trainee a position and then change their minds because they can't start immediately. Graduation next year or year out, it don't matter. Apply now and be honest about your circumstances, but not
too honest about the presence of any alternative plans. NATS wants commitment rather than "s'pose it'll do for now" from it's trainees. They do, after all, spend a whole bucket of cash on them.
Better to find out now whether you have the ability to train as an ATCO rather than have your plans buggered up after a year of anticipation. Best of all, if you fail selection this year,
you can re-apply next year. if you wait, you'll waste another year kicking your heels waiting for your second go.
America isn't all it's cracked up to be anyway. Bournemouth is much nicer