Hey Anon,
I'm really sorry to hear you are going through this and I hope you are feeling better now.
I am a third year student and I submitted my undergrad dissertation in March.
It is not uncommon to get an extention on your project, pretty much everybody I knew had an extension because they simply just asked the teaching office and got 2 weeks extra.
It is a little late for you as it is due today but it might still be worth asking as you have a valid reason.
Otherwise I believe it is counted as any other module (with a higher weight I'm assuming) and a late submission by a couple days results in a degraded mark. ie your A becomes a B.
I am not sure about resitting but my dissertation grade entirely depends on the viva that is followed. If I am to miss my viva I get a 0 in my dissertation and it affects my summer graduation. If you are eligible to graduate this year, you should have received an email outlining what credits are needed to graduate per module and how dissertation affects your graduation. Here is what my email (for BSc Hons) says:
"If you fail a module but the final module mark is lower than 7.0, we cannot condone this and you will be expected to attempt a re-sit for this in August, meaning you would miss graduation. Unfortunately there is nothing we can do about this, this 7.0 target is a university regulation that all departments are expected to implement.
Please note the above doesn’t apply for your Third Year Project, you must pass this module as this is linked to BCS Accreditation. You will be expected to re-sit this if you fail. " In my case, "third year project" refers to dissertation. Please note that the above quote might be specific for BSc Computer Science
I hope this helps and I hope you are feeling better soon

Good luck !!
Vyomika
LU Student Ambassador