I'm sorry for your panic attack, Lemon. I had issues with those during class when I was in grad school, and it was a real struggle. It's good you reached out to the psychiatrist. Maybe he could put you on a cancellation list? Not sure if those are a thing in the UK (or have a different name), but basically where the pdoc would call if someone cancelled. Might be last-minute, but I assume it would be a virtual session, so you probably wouldn't need much notice (if you weren't in class, of course). Or maybe he can give you another name.
Another thought could be trying to contact your regular doctor (if you have one)--mine recently prescribed me something for sleep that can also be used PRN for anxiety, hydroxyzine. Or could prescribe something else. No idea if you could get an appointment sooner there vs. a pdoc, as your healthcare system works differently!