Not sure whether to do a gap year or go straight to uni

Hi, I have been on UCAS and I chose a couple of aerospace courses that has the option for deferred entry. I am considering taking a gap year to get some experience and I have applied for one but I am not 100% sure they will choose me. I am wondering if it's possible to select September 2016 instead and call them up when I get my results next year to defer my entry (if my gap year plan do accept me) or do I have to reapply?

Thanks, it's probably a silly question :P