I left year 13 with only two A-levels and also wanted to go to uni so I decided to take an extra year to teach myself an A-level. I decided on Sociology which I'd never done before(also didn't really know what it was haha) but it actually ended up being a really good option. The fact it doesn't include any coursework/practicals/speaking exams etc meant I could study it independently while working a few days a week and just paid to take the exams at a local exam centre.
You can pay to study an A-level online but if you're relatively self motivated I'd honestly say it's entirely possible to teach Sociology to yourself. I bought the textbooks and revision guides second hand on eBay and used Youtube and thesociologyteacher website to look at model answers etc, and I got an A* so it's definitely possible!!
One of the A-levels I did at school was French, it was my best GCSE subject by far but the jump between GCSE and A-level was big and I found it really difficult. In my experience it's a subject where you need more support from teachers/language assistants to understand where you're making mistakes especially with the speaking exam, so it's not the easiest to teach yourself/study online. That said, if there was a possibility to do it at your school then I think you could do it in one year, it was just a much more labour intensive subject in my experience !