Should I focus on my extra-curricular or academic achievements?

There is, unfortunately, no easy answer to this question! Different universities look for different things, and the best way to get a feel for what they want from you is to attend their open days and look online to see if they have any guidance!

Personally, I know from my experience with Cambridge University that they tend to prioritise academic achievements and areas of personal study over extra-curricular participation. There is so much room for your application to stand out, as well as to set up your interview in a way that you can pre-empt. You can do this by configuring your personal statement around a set of personal projects that you spend about a paragraph each on, going into depth and demonstrating your skills in your subject and your passion for it. By doing this, you also set up clear areas of competence which at least one of your interviews will most likely focus around, allowing you to prepare for future stages in a more precise manner. A great way to get started on one of these projects is an EPQ, which will provide you with a formal qualification for your efforts. Beyond that, I strongly recommend pursuing the areas of your subject that your school has never broached, but you know you want to cover. The very ability to balance this against A Level work will show your competence as a candidate.