I'm not entirely sure, but I am pretty sure most science A-levels will require a GCSE science; If you have one particular science in mind, you could try to pick your ideal exam board (Examples are: Edexcel, OCR, Cambridge, AQA). I suggest having a look through the syllabuses and research how the exams are taken; You may prefer to do an IGCSE. Then, of course, I suggest studying and taking the exam.
Depending on the exam board, the exams can be sat more the once throughout the year; I believe it is possible with one of the exams boards to sit the exam in October/November, depending if you don't want to pay the additional late fee of sitting the exams in the summer, and don't want the stress of cramming GCSE science study within a month.
I'll try to do a little research to see if I can find the exam board that does that (baring in mind, that it may not still be possible), if you are interested? 🫶😊
Another potential solution, if you wish to start A-levels in the upcoming academic year (and are up for a challenge), you could try studying through past papers and learning how to answer the questions, you'll also pick up the required information, though your understanding may be a little hole-y. This solution is mostly based on the assumption that you continued learning science previously, I would highly not suggest doing this if you have little understanding on the subject.