In any disciplinary issue, you are entitled to be given at least 24 hours notice of any such meeting, and they HAVE to advise you of your rights. Your rights are that you are entitled to have another impartial person present such as a fellow worker or a union official.
The notice which they give you has to contain (legally) information relating as to WHY you are being disciplined and the reasons for it - i.e. what you have done, and why it has resulted in this course of action (i.e. breach of contract, gross misconduct etc). So it amounts that you must be told exactly what you have done which constitutes such severe action. However, going to disciplinary measures without discussion is unusual, but not completely off key.
If "Come with me for a sec" turns into a disciplinary-style meeting, as said above you are entitled to halt it and advise them you do not wish to discuss this matter without the appropriate procedures taking place.
Tomorrow is your chance to have documented your response and everything which is said, which could prove useful in the future. I hope the following comes in useful:
DO keep calm and polite in all your responses
DO explain the circumstances if they help explain your actions
DO tell them why you felt your actions were justified
however!
DON'T show negative emotion such as anger, shouting, raised voices etc
DON'T say anything which admits something you do not believe to be true
DON'T try and make unjustifiable excuses in order to "get off the hook"
More importantly, DON'T STRESS yourself over this matter, it is simply a meeting to discover all the facts and to document everything. I have attended disciplinary matters which seem quite clear cut, until other circumstances have been bought into the room which were unknown at the time, only for the meeting to result in nothing more than a "please refrain from doing it again".
Good luck, and as said above - you can appeal! After tomorrow, I would consider getting in touch with Acas -
Acas - How can we help? - who are very good at helping employee's who are confused as to their rights and who have problem employers.