In my opinion, you should do A-Levels to widen your options and to have some time to think over being an Accountant. By doing other subjects, you're exposed to new areas of expertise, and you never know if you might start to really like one or more of them. After the two years, you can still do the Accounting course if you so choose, however if you do the course now, you're stuck on the path of the Accountant.
If you do A-Levels, make sure you check the requirements for the Accounting course (are there any age restrictions or required qualifications). If there are none, pick the subjects you will enjoy the most. I recommend for Accounting to be one of them, just to see how much you truly enjoy it. Then, the rest should consist of the ones that compliment Accounting (like Maths, Econ, Business or just any science). Make sure you ENJOY THEM though. Don't suffer through an entire year. I suggest you join the Sept intake and not waste any valuable time, as you will be behind on:
1. Making friends
2. The content
3. 6th Form life
If you come out of your A-Levels and begin your Accountant course and hate it, you will have those A-Levels in your back pocket to be able to get a good job or, even better, go to Uni.
Personally though, I think that Accounting is a mind numbing and meaningless career, one which the true materialist pursues. Meaning is much more important than money; money does not lead to happiness. I've heard of plenty of dead-inside lawyers and accountants who are always busy getting more money but having no personal, spiritual or family goals. However if that is the career you choose to pursue, so be it.
Hope that helped, if you want to discuss it some more or something, let me know