'I didn't know' or 'they didn't tell us' is not a defence and indeed will not be a defence - of course plagiarism rules extend to exams, it is incredible that anybody thinks they wouldn't. It is incumbent on the student to know plagiarism rules and if you're rewriting chunks of material effectively word-for-word, then you have to cite it, exam or coursework. It has always been thus.
In the other thread, you say that you were explicitly told that you did not need to reference. which is true? That you didn't know, or that you were told the opposite? If you were told the opposite by a member of staff then you might have something approaching a mitigating factor, though the outcome is still relatively unpredictable. Yes you have previous good academic judgement etc, but this is 3rd year and 3rd year penalties are in the main automatically much harsher.
You should be in dialogue with your student union about this and if you are not, I advise that you make this move quickly.
Lawyers increasingly tout their services in these areas but the reality of the situation is that they have no clout in matters of academic misconduct - it is an internal, non-legal matter that cannot be litigated. All they can do is offer (expensive) 'advice' on how to respond. They might be able to help after the fact if there's a breach of contract claim to be made, but that's a different thing. Generally, the SU can do the same thing for free.