Based on how difficult many people say ATPL Theory is, I can't help but think it's going to be a big jump from GCSEs. The benefit an airline would get from demanding reasonable A-Level (or equivalent) results is that they would get people who had proven experience of learning a lot of theoretical information in a short space of time.
I'm sure there are quite a few commercial pilots out there with only GCSEs but a lot will have had serious amounts of life experience before heading down the modular route or similar.
Seems Ollie has hit the nail on the head unless the funding situation changes. Some airlines seem desperate to attract people who haven't previously considered flying as a career, yet there's already bucketloads of people with the talent and motivation but sadly not the cash.