If the police have reasonable grounds to believe you did this, I would have though (depending on your back ground) you would be given a fixed penalty notice for disorder - drunk and disorderly (I assume you were drunk) or criminal damage.
But to be honest, it is slightly irresponsible to throw items off a balcony as it clearly poses a risk to people beneath, and because of this you could get a more serious penalty - which may be a caution. I would be slightly surprised if you were to go to Court but it is a possibility.
If you get a PND: no admittance of guilt, no criminal record, no conviction.
If you get a caution: admittance of guilt, criminal record, no conviction
If you get convicted in court: criminal record and conviction.