Start your supercurriculars early, be it reading books, participating in challenges, etc. The earlier you get started on this, the better your personal statement.
Make sure you get the best GCSE grades you possibly can. Should be aiming for >=7 in all subjects ideally.
Choose your A-levels wisely. Make sure to pick further maths if it is available, and self study it if it is not.
After your GCSEs are finished, invest some time in learning the basics of different areas in computer science (cyber, ai, quantum, theoretical, etc.) and make sure you are confident with programming.
Other than that, have a very restful summer holiday when it comes. After y12 starts there will be less opportunity to really slow down and enjoy things day-to-day, so take full advantage of your holiday.
Best of luck