There is no JAA or CAA requirement for GCSE's. This has probably been imposed by the schools who are required by the CAA to make sure you can cope with the maths and English requirements of the course. We often get students who are slightly behind the curve in maths and suggest either a local college course or local private tuition to get them up to scratch. It really does save time in the long run.
If you can demonstrate adequate mathematical ability and communicate effectively in both written and spoken English they should take you whether you have the bits of paper or not.
This only applies if you are paying for your own training. Courses that involve airline sponsorship are more rigid. Then you do need the bits of paper to even be considered, they use the exam passes as a filter to reduce the number of applicants. In the process they undoubtably ignore many very capable candidates but they have so many applications they can afford to.