Dude, he cannot touch you for what you did when you finished on time that day. The only problem I forsee is if you have other issues with your work performance. Thats the only angle that he can rightfully tackle you on. If he does tackle you on other issues then you could just say, "Sorry Guv', I will try and improve my performance".
If, however, he tackles you only on what happened the other day, you have absolutely nothing to worry about. Be as calm as possible and just explain to him that you were ordered by the manager to stay. You were treated like a child, which you took offence to. Hence, you decided not to stay behind. Also tell him that in future if he wants you to do extra time, to give you as much notice in advance as possible, as you do have other things to do, eg. study for important tests/exams as you are aiming to get a top grade.
Like I said, you have nothing to worry about if they have nothing else on you. If he calls you in and gives you a warning based on what happened the other day, you should have a big grin on your face, because you will lodge your appeal later this week and in a few weeks you will request a written apology from this "head honcho", for not only giving you a warning wrongfully, but also for the fact that his manager kept you locked up in the shop when you wanted to leave (imprisonment). If I were in your position, I would love it, if they gave a warning for what happened that day. You could even book off with stress, get a doctors note - take a couple of weeks off. The possibilities are mind boggling...
Oh and it could be that he just wants to speak to you. You wont need a witness for this, but if he decides to give you a warning, take someone in with you. Make sure you do this.
Another thing, my feeling is that you are worrying about getting sacked - what you did that day is not gross misconduct which means that he cannot sack you for that incident alone.