Bear in mind that there are plenty of self-funded pilots flying for airlines who do not have good school grades, but also bear in mind that times are changing and in the current market employers can be more choosy. The thing that counterbalances lack of formal education qualifications is hours and flying experience; for sponsorship you won't get a look in with 3 GCSEs and nothing else, but if you have a few thousand hours of jet time on the type they fly, most will call you to attend their selection for direct entry should they be recruiting, even if you only have 3 GCSEs.........
PP