Do your A-levels, it will open up your options quite significantly, especially in this day and age. Some airlines don't accept students with no A-levels; BA FPP requires A-levels or a Uni degree on top of your GSCEs. It doesn't hurt to do it, and you can use the qualifications gained from school to help get a better job to get experience that these Airlines love to hear during interviews. If you do decide to do it though, make sure you choose what you want to do, rather than just subjects that you feel will help.