Sempre e un piacere vedere altri italiani nella TSR
I didn't take Italian at A level because for reasons that I won't get into, we stopped speaking Italian at my home when I was 10. I did however take Spanish A level because that was my other mother tongue, if you will. If you are fluent, probably even conversational in Italian, you can probably just take the exams with very little preparation. Obviously practice the exams but I found the Spanish exams I sat to be incredibly easy and I barely did any revision.
However, two massive caveats:
1) One of the exams will require you to write two small essays on a book and a movie from a predetermined list, so you'll have to watch a movie and read a book and know them both inside out and practice essay writing and learn a lot about the context behind both. So for this, you really want some lessons/tutoring to make sure you've got it, and you really need to study the movie and the book and practice it.
2) the speaking exam will require you to present a topic relating to Italy. It can be any topic of your choosing, but you need to be able to speak about it for 5 minutes and then have a conversation with the examiner about it for another 5-10 minutes. Then you do some speaking card things, really easy. The presentation bit is a bit tricky, you do need to do an awful lot of research and practice and the examiner needs to be able to ask you questions about it (in fact I had to fill out a form where I suggested the examiner a few questions he could ask me!). For practicing the exam, it was really helpful to do run-throughs of the presentation with the speaking assistant at my school, although I assume any old family member or friend who speaks Italian will do - I did plenty of practice with my friends and we mutually benefitted from it.
I think you can take it on your own but you will need a tutor to address the caveats I said above. Alternatively, I think if you can find a school that offers Italian A level, go for it. Yeah, most lessons will probably be a bit boring for you but you'll benefit from them anyway, and it's just more fun that way!