Hi lovely, you are able to withdraw from an A-Level right up until the point of the completed exam papers being sent off. I withdrew from an A-Level a week before an exam. This was due to health reasons, and my college was supportive, so I appreciate it was in different circumstances.
Whether withdrawing from that A-level is the right decision really depends on how many other exams you are taking, whether the grade you get affects your university offer, and how withdrawing or now withdrawing will impact you. Have you spoken to a tutor about this? Please know you always have options, and there are foundation places and many courses in clearing, if you don’t think you will make your offer. A “bad grade” is subjective too - if it is still close to your university offer, I would talk to the admissions department and see if they will make an allowance. This can be common, depending on the university.
Good luck with your exams and future study!!