Ok, let's be realistic here, it's better to be brutal than kind when giving advice on this matter, simply because of the scale of money involed. . . Firstly, let me just say this - If you have the right attitude and are a patient person, you can achieve what ever you want.. . However, in reply to your question, you must ask yourself three questions :
1. Since you only have GCSE's, how confident are you that you would be able to deal with the material presented on an ATPL course. Everybody likes to think they're clever chappies, get your hands on some of the material you'd be expected to learn. My first degree was physics, so I'd have a good understanding of subjects like aerodynamics and thermal topics, on the other hand, these can be quit difficult if new to them.. .Don't be put off by topics with big names or topics that sound difficult, if your prepared to study hard, then there's no reason why you cant do it.
2. Say you get your ATPL! (yeh!!!) So've you've studied hard, and spent lots. Consider the compition between you and all the others with ATPL's, except more than likely, they will have more than just GCSE's. . . Getting that job you dream of will involve at lot more than just presenting your licence. They'll compare you with others, and yep, qualifications will come into it at some point! Who knows though, you could be the best interviewee there! Who's to say what qualities/qualifications they look for...
3. Finally, don't forget the old saying - 'right place at the right time...'. So you've got the licence, the qualifications may/may not be irrelevant, after all that, you could just end up in the same position as many pilots - always looking, but nout' happening!!
Hope these pointers help! Just read loads of pilot mags/checkout pprune/get to know some wannabes etc...
It's a big decission, but one well worth it i'm sure if you make it!
All the best . .G-HIGH