Here's what I think has happened (I do this every day):
The University are already aware of mental health issues from prior MCs and so accept that this is an ongoing thing. Presumably, you told them this previously, within the deadlines for MCs. Tis is good but it has also shot you in the foot a little, as I will explain.
You've told them once, on time, about mental health issues. So from the assessor's perspective, you should have told them on time this time too, and not left it so late. This is actually the standard line at basically every institution in the country. This is likely why they are saying it can't be used as a reason for the late submission rather than 'it can't be used as a reason for an MC claim' more broadly. Getting retrospective MCs is incredibly difficult. I've seen it happen in places I work about 4 times in the past 6 years.
If you were/are final year, withdrawal is generally a possibility for failure of a unit. I don't say that to scare you, but to know what you might be facing. You need to get onto this ASAP. it is also possible that they work something else out, a retake year (with or without attendance), an exit award etc. But you should know all possible outcomes.
The sticking point for me is really that they appear to have accepted them and then changed their mind. This, if accurate, is highly irregular.
Did the AA/PT say they had been 'accepted' or that they had been 'received' (or similar) - there is a difference and it is not usual to have AA/PTs directly involved with the MC panels. A lot of the time, AA/PTs are not even directly notified of outcomes.
If it is accurate then you might be able to appeal something on the grounds of the mark itself, administrative error or procedural misstep. To do this you really need to get your head together with the Student Union. Your AA/PT usually can't help much with this because it's a conflict of interests.
For steps you should take, I'd speak to the
SU (I believe at Brunel it is ARC that you need).
I'd be
reading the Appeals literature and I'd particularly note Paragraph 9 in order to claim for an extended timeframe for your appeal. The regulations can be 'reasonably adjusted' to allow for criteria in the Equality Act 2010 to be satisfied. I'd try to push this request because (this depends on a diagnosis) you have a long-term mental health issue which satisfies the criteria set out in
Section 6 para (1) Equality Act 2010:
A person (P) has a disability if—
(a) P has a physical or mental impairment, and
(b) the impairment has a substantial and long-term adverse effect on P's ability to carry out normal day-to-day activities.
As per MIND, you need to demonstrate the following to class your mental health as a disability, and you do that by demonstrating that it:
has more than a small effect on your everyday life
makes things more difficult for you, and
has lasted at least 12 months, is likely to last 12 months, or (if your mental health problem has improved) that it is likely to recur.
If you can demonstrate this stuff, then my gut instinct is that they might be open to reconsidering their initial position - it is worth reading this and seeing if you can bring it to bear in your favour. You will likely need the help and support of a GP or other mental health practitioner. I've seen private therapist letters help with stuff like this.
Apologies for the wall of text, and best of luck.