I did an extra year. I have autism and walking difficulties and as soon as I entered sixth form, any previous support I'd had disappeared. They suddenly expected me to have no issues at all, now that I wore a different colour blazer. I understand independence in SEN students should be encouraged, but a smoother transition would've been preferable, to say the least. I got 9A*s and an A at GCSE, then got D, U, C in AS Biology, Chemistry and Maths respectively. Honestly, the way the school was acting towards me was straight up discriminatory. I had surgery on my hands and I didn't get any access arrangements in exams to help me write, or permission to type exams. By the end of exam season, my hands were useless and spasmed so much that I couldn't even hold a pen due to overuse.
I transferred to a different college and things were much better. They didn't even do that much extra, there were just more safety nets in place, so to speak. They gave me a lot more independence where I wanted it, and more support where I needed.
I studied AS physics, biology and chemistry. I also continued maths to A2, because I'd got a C, which allowed me to do A2. I was basically a normal AS student, with slightly harder maths. I also resat my AS maths units. I also decided part way through the year that, seeing as I'd be finishing A level maths that year, I wanted to do Further maths in my second year at that college. I had to self study Statistics 1, because of the way this college structured the maths/further maths.
I got all As in my first year and an A* in Maths. I applied to and took part in the UNIQ Biochemistry Summer School programme, which was great, because it gave me plenty to talk about in my personal statement; There was nowhere to go for lab type work experience anywhere near where I lived. In my A2 year, I applied to study Biochemistry at Oxford, Imperial, Sheffield, York and Durham. I got offers for all of them and decided to go to Oxford. I am now sat typing this in my new accommodation in Oxford, waiting for Fresher's week to start on Monday. In the end, I got A*s in Biology, Physics, Maths and Further Maths, with an A in Chemistry.
So I will say, don't give up hope if you are doing an extra year. Oxford look for potential and enthusiasm, and a bad year doesn't mean you don't have these things.
Good luck!