You can go to prison for GBH, however if you didn't mean to do it, this may be taken into consideration.
You say you did not throw the bottle at her, and yet it hit her back? So you meant to throw it at her, you just didn't mean to cause harm. There is a difference between GBH and GBH with intent, the latter being the more serious, so you would have to convince the jury that there was no malicious intent to cause GBH. This is very important.
There is a section 18 and a section 20 that cover GBH, the punishments for these differ largely and it all comes down to 'intent' to do harm, and whether you can prove this in a court of law. Section 18 requires intent, section 20 covers the harm done to a person from another and does not cover intent.
If you have convicted of a section 20, then the maximum sentence would be 5 years in prison. Or you may be fined, and not go to prison. A section 18 is more serious due to the intent to cause damage and you could go to prisn for any period the judge prescribes.
You must argue your lack of intent to cause GBH.