Hi, im in my gap year studying another a-level online. I don't see why you cant apply privately while at another college, just make sure the extra a-level your thinking of studying is worth it. Having 4 A-Levels wont benefit you unless you are trying to meet entry requirements. Also, if you were planning to sit all the exams next cycle in 2025, its going to be very difficult to learn a whole a level in 7 months and do your current subjects. You could do the a-level in a gap year like I have done if you think that would be easier.