Language certificates doesn't really match language abilities: for example, I don't even have FCE.
Each of the ESOL Examinations chcecks only language abilities and nothing more. By the way all of them are waste of money. No specific vocabulary is being tested. Approaching FCE, CAE, etc. you are supposed to fill in gaps or match other shit together in a context. To simplify, ther's no way to evince your konwledge in english - polish translation.
I wish I would be some kind of help for your GPL scripts, however I am not certified too.