You could do the GCSEs in two years and the As in two, applying before you get your A2 results, perhaps. The RN allows you another 2 years:
Aircrew Officer (Pilot and Observer)
Age on entry 17–26
Nationality British or British/dual citizenship
Qualifications At least 140 UCAS points and 5 GCSEs (A*-C)/ 5 Scottish Standard Grades (1-3)/ equivalent, including English and maths