Hello fellow Brummy
I've applied to KE CH + FW + A. Handsworth, and Solihul Sixth Form. Didn't apply to JC, just didn't like the atmosphere there. That's just me I guess.
My friends have had interviews at JC already, and trust me. It's nothing to be worried out. They practically have your name printed out on the acceptance letter already. They just want to see you in person really, maybe answer basic questions about why you want to come here. Unless you seriously irk them, you'll get a place automatically.
Hope that helps. I'm a worrier too and I know what it's like to be in that sort of situation. But honestly, this interview is nothing to be worried about, truly.
All the same, best of luck.