Perhaps I've gotten the wrong end of the stick here, but from your initial post this doesn't sound like the issue was that you weren't given an opportunity to properly quote your sources, it sounds like you've amalgamated several pieces of work not belonging to you, to look like a unique piece of work. These are two very different things.
The first is someone inserting something that is clearly a quote from elsewhere, to support their own work, but have either forgotten to or not been able to properly reference it and therefore been penalised as a result. This is harsh, but happens and is a valuable lesson.
The latter is someone cobbling together multiple other people's work and passing it off on their own, which is pretty much the entire definition of plagiarism. This it totally deserved and it should be common sense that this isn't acceptable. This would be immediately picked up by the exam board's anti-plagiarism software (if not by the teachers reading it).
As I say, if it's the former then possibly the teachers have something to answer for, by not informing you how to reference properly. If it's the latter then it is obvious why this has happened - it isn't your work, regardless of how much effort putting together the different sources was. I have sympathy for you, but really I think you should count yourself as having gotten off fairly lucky given it will not appear on any record and you can still pass without this assessment.