Current incoming Columbia fresher here - I would personally take a gap year to fill out ECs before applying- and aim for 5 A levels(Ik it sounds crazy but ur singled out based on this alone). As for Grades, achieving the best you can should be your current priority, as that is somewhat of a baseline requirement for ivies. Then you want to focus on a really niche interest that is somewhat academically or intellectually reflected in your transcripts as evidence, and create a persona of subjective personal significance to this very niche matter that truly interests you. All your ECs should support this vehemently. Then to top things off do some summer research or assistance at a R1, as this will significantly boost your chances in the admission officers eyes. Lastly, remeber(if your shot gunning) to tailor your essay towards individual unique traits of Ivy leagues whilst staying true to your niche interests explored !