Get your supercurriculars out of the way early. You should hopefully already have done some by now. Aside from other activities, your supercurriculars must include at least 1 or 2 things which demonstrate your mathematical ability, and some things you did which demonstrate your interest in computer science specifically (like coding projects).
For the entrance exams, theres not a whole lot you need to be doing right now, although you must ramp up at the end of y12. At this point just make sure you are confident with all of the standard A-level content you have covered so far. If you are feeling especially keen you can try doing some TMUA questions from earlier years (don't try anything remotely recent - you need these closer to the time). Alternatively, you can try getting through the STEP foundation modules or just doing lots of senior maths challenge questions to build on your mathematical thinking. Generally remain engaged with maths.
Oh, and make sure you get your A* predicteds. Your chance of admission drops precipitously with more than one A.
Best of luck for your application.