With ADHD, the most important thing you need is organisation and support (by school, medication etc). It's hard to do all of this by yourself, so please do reach out to your school. And, definitely drop 1 A level (any).
The problem about university is that in the more traditional ones like Oxbridge, LSE, UCL etc, you'll be doing the majority of work yourself. Everything is reliant upon self-study and self-research. That's how it was for me at Edinburgh and all my other friends at Oxbridge or other London universities. It really is tough and you will struggle more than your neurotypical coursemates. Even so, it is doable.
To be honest, if I could go back in time, I know that I would prioritise my mental health over trying to do everything and anything. I kept ignoring my ADHD and it's really affected me long term. I'd change a lot of things like admitting to myself to take a year out in order to get to grips with ADHD; by asking for help and not being stubborn or too proud in case I appeared weak or lesser; by taking extra time for exams and taking extensions; or, by acknowledging that something was wrong seeing as I would always do everything last minute (the night before) all the while panicking myself into hysterical heights. Each time, I'd swear to myself that I'd do my best next time and that I'd do it in a timely manner; yet, that never happened.
I'm 26 now and it's only in these past 2 years have I been able to get to grips with my ADHD and be able to give my assignments and everything else my 'best'. I don't do important things last minute either, and I certainly don't do any all-nighters.
In other words, I'd advise you to be cautious and practical - if you take on too much to reach your 'potential', you may just end up taking too many steps backward, thus impeding your progress for the next several years. This will be irreversible; please do heed the advice of someone who was also crazy enough to do 5 courses in his final years of secondary.
I promise you that there are multiple routes; you're not going to forsake your potential just because you're not doing everything in front of you right that second; and, you're certainly not below others either. You have the potential, and you can reach it if you're patient and kind to yourself and your present capabilities. You have to take small steps and slowly find what your limits are as of now and then build upon them over time.
Ironically, you can't rush this ADHD journey even if your affliction tries to do so otherwise. Patience and faith.