I may want to cancel my uni place, also can I apply abroad with a firm offer in uk?

I have accepted my firm and insurance choices for university deferring until next year (Sep 2017) however I am going to apply for a university abroad in the beginning of 2017 and if I get the place there I'm going to accept it. Now question is, can I cancel my uni choice on ucas next year if I am accepted abroad, how do I do that and when is the latest I can? Also the abroad university I am applying to doesn't go through ucas it is separate so is it ok to hold a firm offer on ucas for the uk uni at the same time as applying abroad incase they don't accept me? Thanks :-)