No matter whether or not you eventually get into the RAF, having a few more GCSEs at good grades will be helpful in any career you want to follow. Rather than wasting effort trying to get in with less than the minimum requirement, it would be better and simpler to try and get a few more good grades under your belt.
I don't know why you only got a D for Maths (perhaps you missed part of the course due to absence, didn't bother to revise, etc), but required subjects are required because you need a basic level of aptitude in order to be able to do the job. So, if you're rubbish at maths, you might struggle with parts of the job. If it's just a matter of putting the work in to get the grade, why not put the work in and then resit?