Failed and withdrawn from uni, mitigating circumstances rejected.

Did you provide all the death certificates and a doctor's note stating you're being treated for anxiety and depression? If so (and I'm no expert on these matters), I would think they are being unfair and that you should appeal. Chucking someone out just because they have had a bad year isn't really on if they have mental health problems

With regards to how to appeal, there should be a paragraph about how to do this in your uni's handbook, if you were given one when you started. Otherwise approach your SU, who can advise you further and send representatives to meetings you may have to attend

I did not provide any death certificates because they were difficult to obtain (they are abroad), and honestly I thought a letter from the uni counselor was sufficient. I guess I was wrong.

I am also seeing an external therapist so will ask a statement from him as additional evidence and i guess I will have to ask my parents to obtain the death certificates. I didn't want to ask my parents initially because I didn't want them to worry!

Can totally understand why you didn't ask your parents. I'd def go to the SU and talk to them about this and in the meantime, gather all the evidence you might need. It may be that the letter from the uni counsellor wasn't enough So maybe try and get the death certificates if at all possible. Also talk to your university's disability office - depression and anxiety come under mental health disabilities and it's illegal to discriminate against you based on that

I did not provide any death certificates because they were difficult to obtain (they are abroad), and honestly I thought a letter from the uni counselor was sufficient. I guess I was wrong...

What you have to bear in mind is that those who are responsible for considering things like deferrals and mitigating circumstances on behalf of your University will be bound by regulations and guidelines as to what you, the student, should provide by way of reasons and evidence and when these things should be provided. Usually what they look for is something that amounts to 'independent corroborating evidence' which supports your reasons which is why things like death certificates are usually sought. Provided the decisions made by your University in relation to you have been consistent with their regulations and guidelines you probably won't have a strong case under appeal, but don't let that stop you from making one. As others have suggested, look for advice from your Student Union and maybe try to familiarise yourself with whatever regulations were applicable to your requests for deferral and/or mitigating circumstances.