Choose a role that you think you will be most suited to and will enjoy, and then get the educational qualifications for that job. If that is RAF mechanic, then you need those qualifications. No way around it. You can have a PHD in Zoology, but if you don't have a C in GCSE maths and English, then you won't be accepted.
Regardless, I would still advise getting 5 GCSE's A- C including English and Maths, even if the job you wanted needed no educational qualifications. There is no rush to join, jobs in the RAF open and close all the time and unless you are a man in his 30s (which I'm sure you're not), the opportunities will be open for you for a long time.