Can I redo an A Level in one year?

I think I'm going to miss my uni offer by missing my grade in one of my subjects. If that does end up happening, I want to stay at home and resit that subject, as the uni course I've applied to is the only one I'm interested in doing. Would I be able to resit the A Level in one year, or would I have to redo the whole two years? My parent has said they'll support and fund me in resitting the course at home instead of at a sixth form or college if possible, as I find that environment really stressful and it makes my chronic illnesses worse. I don't think I'll need the whole two years to improve my grade, as I've understood the content, it's just that my ability to study has been impacted by my illness.

you can resit privately as a private candidate. You need to use JCQ to find a centre near you that offers this and pay the fee for the exams. This needs to be done usually 6 months in advance. Resits are once a year (May/June for a levels). You do not need to repeat year 13 if you are a private student.