Hi, it’s totally possible! I know because I did it - went from BBC to A*A*A in the end (and got my grade 8 violin in the December of year 13). Admittedly I did start working during the summer holidays, but there’s definitely time. Break all your topics up into smaller individual tasks/subtopics and the put them into checklists, doing a couple of things for each subject every day and you should end up powering through it! My main tip would be to get organised, and fast, as then you have your plan ready for the year and you can focus on what matters.
Work smart, rather than for a long time as well. Don’t say to yourself “do this subject for two hours”, as for me that’s how I got burnt out really quickly. You can get even more done if you just focus on the tasks themselves rather than the exact amount of time you want to be spending on them. Take breaks regularly (whenever you need them - don’t force yourself to keep working as you’ll get burnt out and create an association of boredom, tiredness and lack of progression with studying which will ultimately lead to you hating it!) and don’t lose your social life (at least not until the last month before your exams haha).
Try to spend less time worrying and more time doing, as that’s the only way you’ll get it done. Once you make a start it’ll feel a lot better - trust me!!
What are you hoping to do at uni and which uni?