An investigation will be carried out and then, depending in the outcome, you'll be invited to a disciplinary hearing. There will probably be an investigation meeting beforehand which you'll be invited to
The investigation meeting can happen at any time, without the need for prior notice unless your disciplinary policy says otherwise. Which is quite unusual. You don't have the right to be accompanied at that meeting.
If your employer decides you are sufficiently sorry, they may give you an off the record warning and leave it at that. If they want to give you a formal sanction (including a verbal warning) or dismiss you, you'll be invited to a disciplinary hearing.
You'll get notice of that in writing and of the evidence your employer is relying on. You'll also be able to bring a tu rep or fellow colleague to accompany you. They can't answer questions in your behalf but can ask questions and clarify things.
After that meeting, your employer will write out to you with the outcome and you will have a right of appeal.
The sanction you get will depend on the environment you work in, whether you've done this type if thing before (or have precious live warnings) and whether colleagues have been dismissed/warned for dpi g similar.
I'm an employment lawyer and I would expect you to get a warning for this rather than dismissal but it depends very much on what your job is and whether confidentiality is important.
Your best bet is to admit what you've done, apologise and be very contrite about it.
I don't want to be unhelpful or kick you when you're down but you shouldn't be looking at documents on other people's desks and, if you do see something you shouldn't, you need to keep it to yourself.
Your employer should have a disciplinary policy which is compliant with the Acas code of practice on discipline and grievance. You can google that code and check it is compliant. It also has helpful explanatory notes which will explain the process.
If you need more advice, you can call the Acas helpline on their website.