Well, where to begin?
Firstly, it is an expensive and time consuming business embarking on flying training. The good news is that there is no particular educational achievement level required to embark on any flying training, towards any of the various types of pilots licence. People will come on here and tell you how they couldn't pass an exam to save their lives, but are now criss crossing the globe astride their 747's with not a care in the world. In a few cases that is undoubtably true, but such achievments were almost certainly the product of a different time, and certainly involved an unusual degree of luck. The reality in more recent years, is that competition is very tough, and where vacancies do arise, it is important to have an arsenal of attributes that make your curriculum vitae stand out from all the others.
Some methods of training and certainly all forms of sponsored or subsidised training (and they are almost non-existent at this time) will require a good educational certificate portfolio. That would likely be at least 5 GCSE passes at good grades to include English Maths and a science based subject with a preference for physics. Many would also require at least 2 good grade A levels with a similar bias towards mathematics and a science.
Airlines that do recuit low houred pilots will rarely find any shortage of candidates, and the process of selecting those who will be invited to interview, will almost certainly include an appraisal of educational achievement grades.
It is also worth considering the fact that the study programmes for the various grades of licences as well as the continous learning syllabus as an airline pilot, involves a significant level of knowledge and application of those basic subjects tested at GCSE. Particularly, English, mathmatics, physics, geography, biology, and chemistry. It is difficult to envisage many current airline pilots having undue difficulty achieving a good GCSE attainment at any of these subjects, with a degree of study.
So why are you having difficulty achieving better grades? If this is simply a case of application or distraction, then that is something you need to rectify. There is nothing stopping you redoing these examinations again, or at any point in the future, even through a course of correspondance or home study. If you want to "live the dream" you must be prepared to accept the harsh realities, and also be prepared to put in the required level of effort to achieve that goal.
You are 17. My advice would be stay at school and resit the examinations you need, putting in the effort and resource to get significantly better grades. If you cannot do this, then find a job that enables you to pay for a private flying course, and use your spare time to embark on a couse of home study, to re-sit the important examination subjects in a period of 12 to 24 months. Invest the money at a very early stage to obtain the CAA class 1 medical certificate. Without this your ambition would be severley curtailed. It may well be that the exposure to PPL training will spur you on to improve your own chances, and give you the motivation to take some serious decisions and options. Conversely it might make you realize that "the dream" is a lot glossier than the reality would ever be.
Do I think GCSE grades B,C,DDDDDD,F,G would get you an interview with the like of BA or QANTAS. Not really no.
Do I think GCSE grades B,CCCCC,DD,F would get you an interview with any major airline? Not really.
However you are moving in the right direction, and if you take on board advice that you garner from good sources including your own parents and your own research, there is nothing stopping you from doing something about it, and still enjoying yourself in the meantime.
Good luck with your results when you do get them, they might be better than you think!