There are three or four people on the panel. They are a mix of military / civilian backgrounds, reflecting the membership of the HCAP.
It's worth knowing your CPL theory. Be prepared to be able to answer random questions about anything covered in the theoretical knowledge.
Know about the HCAP. Know about what's involved in flight instruction; who are the students? What are they like, what do they need, what's the expectation.
Be confident in how to interview in an evidence-based way using examples from your own personal experience.
Prepare your presentation thoroughly. Make it interesting and don't be afraid to make it fun.
The panel are there to get the best from you, so remember that. They're good people with a huge amount of experience, so their advice and feedback is relevant whichever way it goes for you.
Good luck! And if it's something you're passionate about, don't let a set-back this time stop you from following your goal.