Why would you need a gap year to study A-level biology? Buy a revision guide, read it, and then sit the exams. I went the other way and did AS-level chemistry while studying biology as an undergraduate. It took two days in the library with revision books and past papers to score an "A". I'd be surprised if A-level biology took much effort now that you are so much more used to studying and sitting exams.
The only potential difficulty comes if the exam board you are sitting with require a practical assessment. My former school allowed me back to sit the practical components but a local college could presumably fulfil the same role.
Your university don't have to know or be involved.