Do any A-levels you want: it does not matter, you just have to enjoy them and do well. If going to university, try out University Air Squadron and experience RAF life: you may enjoy it and join them, otherwise apply for sponsorships for which you will now be gold dust after UAS in final year along with other graduate jobs, get in...brilliant, otherwise have a good base from a graduate job to self sponsor after a while, and keep applying for sponsorships every year.
Get work experience not just in aviation but useful business areas (airlines are sometimes looking for potential managers in their pilots), do Duke of Edinburgh award and re4lated adventurous training activites, do air training corps if you have time to get gliding experience, work hard and play hard, and with many life experiences you will succeed.