You do not have to have taken any grade exams to join NCO. Your audition is the only thing which will count, so make sure you prepare well to give yourself the best chance. Your teacher will be able to help you.
Yes. This is to make it fair, so everybody has the same chance. Lots of people get nervous before auditions, but make sure you prepare well and try to stay calm. Your teacher will be able to help you. The adjudicators know what you are going through as they are used to doing auditions themselves, so they will be understanding.
Occasionally we are able to accept late auditionees if there is a place available in the Orchestras. This is highly unlikely for instruments such as violin and flute, but more likely for the less commonly-played instruments. We can advise if you let us know the instrument, age and approximate standard of playing. Please contact us.