Your parents should have handled this from the outset. Get them to write, or call, or email, today or tomorrow. Are you sure they even know about all this?
They should explain you are 16 years old and a child, and you have received a demand for £1000. That they are sorry you have caused them so much trouble, and that you are just 16, and cannot possibly pay that amount, and you were just 15 when you committed most of the offences. They should ask that the case be closed, that they have spoken to you, that they have told you how much trouble you have caused, how much stress this has caused them as parents, and apologise very much for all the hassle.
That is what I would if I was your father (and I am sadly old enough to be, I realise!)