Advice on Fit Note request, please

Hi
Last year my partner was migrated from ESA to UC, now before this I had just been on her claim, but now obviously I technically have my own claim for UC as part of her claim.
Anyway, I have just received a request for a fit note from UC, the thing is I've never had a Fit note even though I have stage three Emphysema, Peripheral vascular disease, and cardiac problems......I claim PIP and Carers Allowance for my autistic son who I help look after.
Now as I say, I've never had a Fit note, because I've never been asked before, but obviously now I'm quite worried as there's no way I could physically handle work, mornings are a nightmare where it takes a good couple of hours to get my lungs to function properly etc etc.
I was wondering please if you could give me some advice as to what to do next.
Do I go and see my GP and ask him for a Fit note ?, or do I need to contact the Consultants who deal with my medical issues ?.
Any help would be greatly appreciated.

Hi David
Just checked UC, and on the Payments section it says "You said that Paul is caring for someone" and then there's a figure saying how much we get for that.
Is that the one ?.

Hi again David
Thank you for your reply.
So if I'm entitled to the carers element, does that mean I don't have to supply a fit note ?, as I'm caring for my son ?.
Sorry to sound thick but I don't want to just ignore the fit note request without a good reason to do so.
Thanks again, Paul