First of all, take a deep breath! The presentation is compleatly unassessed. No one is taking notes on you, or marking you. It is a chance for you to go in and get some information from the RAF and decide if it really is for you. It's a chance for them to show you a few videos, take about what the RAF is today and let you ask questions if you have any. (I never did and I didn't feel silly about it) You can go to a presentation and have no obligation to join so they are hardly going to think less of you if you don't ask questions. Just go in dressed smartly, be attentive, ask questions if you have any, relax and enjoy it. They will ask you afterwards if anyone wants to book an interview for AFCO selection and if you haven't told them that already you can do so after. You can back out at any point of RAF selection, even up to IOT.