try to stay calm (she says patronisingly - sorry). Do you have a friend you could meet for coffee before the meeting and walk in to the building with? oh I guess maybe you'll be at work anyway? could you find someone to walk you to the room and then whisk you straight away for a coffee?
remember they'll be as panicy as you - its a horrible horrible thing for both sides (unless they're hellish in which case do you want to work there anyway?)
Just stick to this "I'll talk with my lawyer and come back to you. No I'm not willing to agree anything right now, as I said..." They have to go through the process, even if its disciplinary.
They'll prob give you a letter that states the criteria by which you were selected. Don't feel you have to comment on it because it will feel like an attack on you and your work. Take whatever they give you and go to your solicitor. Try to keep control by saying things like "I assume I can leave for the day now....Good, I'll be in touch tomorrow"
it will be horrid, but less horrid than you're imagining.
CAT me if I can help further.