We engineers are supposed to be the clever ones, and not many of us were straight A students (I certainly wasn't).
As for pilots, a basic ability to read and write is a big bonus, joined up writing is definitely optional. Good maths and arithmetic, to at-least grade-B GCSE standard will definitely help a lot.
After that, it's a combination of attitude, aptitude, and a capacity for hard work.
Frankly whether you want to be an Engineer, a Pilot, or even both - the last is what really counts.
G