I've talked to some past cadets who went on this course and they always tell me that signs of leadership skills are important since they aren't just looking for co-pilots, they are looking for future captains. Also, skills like team work and communication are also very important.
Watching this may give you an idea of what skills they are looking for:
https://www.youtube.com/watch?v=E60dYDaXn08
You'll face an aptitude test of course, so practising mental arithmetic (GCSE level) and using one of them plot aptitude sites that mimic the actual test you'll face to practice would help - be aware its probably not 100% accurate.