I don't think that you can qualify the difficulty of the ATPL's so universally. Perhaps each candidate finds some parts easy and some more challenging, some near impossible,regardless of their ability to pass GCSE, NVQ or other exam previously. It certainly did not feel like a minor hurdle to me, and I have the usual back-catalogue of dusty old school certificates.
There is an organisation called UK NARIC, that determines equivalency of qualifications. Perhaps a call to them may provide a clearer picture than random guesses of how an ATPL series may compare to other tests?