First off, you should really consider what career path you want to follow, and choose a levels accordingly to how many doors they open and which ones might close doors.
Generally, based on your post, if it was a decision between accounting a level and maths a level, maths would be the better choice. Believe it or not, if you have an accounting a level and apply for an accounting degree at university, while it does seem like it will make your chances better, universities tend not to favour it that much, since they cover it during their course anyway. So if you want to do accounting for university, save it for university (but according to your post, you’re going into medicine so accounting a level is pretty useless either way lol)
You already have a medicine oriented selection of a levels, an most universities such as oxford and Cambridge require that you have chemistry, biology and another subject such as maths, further maths, etc, so you’re generally good to go. Maths might be the choice for you here
Best of luck! <3