I know some guys who fly for major UK airlines and they don't even have GCSE's so I wouldn't stress about it if I was you...
You may well, but if you think that is something "not to stress about" then your situational awareness needs significant improvement.
These days every employer sees those same news clips every August, when anxious teens are opening their GCSE and A-Level results only to discover (or so it is often made to appear) that everyone has A-stars in everything. Every year the "question" is always asked "are exams getting easier and easier?"
An employer expects to see education certification that is usually better than average. Young people in particular, have very little substance with which to pad out their CV's, and inevitably their educational achievements are going to play a significant part in how they present themselves.
Airlines get thousands and thousands of applications for every job going. They are only ever going to interview a small proportion of those applications. You can be sure that the filtration processes they employ, will nearly always set out "minimum qualifications." If GCSE's or A-levels are, part of that standard, then an absence of those qualifications will have an application fall at the first hurdle.
Kidding yourself that a "pilots licence" is an adequate substitute for education, is wishful thinking. Whereas, for an experienced pilot other factors may carry greater weight, at the lower levels of the pyramid that isn't usually the case. All interviewed pilots will have the requisite "pilots licence." It is what has made their CV' stand out from the rest of the pile" that allows their application to reach the final hurdles.
You may think your character and charisma will sell you and overcome other shortcomings, but if your CV doesn't get through the filters none of that will matter a jot. Beware those who tell you "they know someone who," or "all that matters is your licence." If you study these forums, or the industry in general, you will see that it constantly evolves. Over the last twenty years there have been major changes in licensing, training, financial risk, and recruitment opportunities. With regard to the latter, those opportunities have become more competitive than ever. Success often lies in those who can adapt to the evolution. Those who think that "it doesn't matter," or "wait long enough and the old times will return," are deluding themselves into extinction.
Back to the original posters question. Do your best! Try for solid results in core subjects. Then look at those subjects that you are good at, or with effort, could be good at, and focus on getting the best results possible.
Whilst straight "A's" in Maths, Physics, and English are always likely to give you an advantage. Provided the core subjects are reasonable, good grades in whatever (within reason) subjects you take, are likely to stand out more than mediocre or average grades in those subjects
you chose to advance to.