From prior examinations of this nature I would guess you need to be to Foundation GCSE standard but have the ability to do it all very quickly as time will be the main problem for most people. Not only are they testing your maths skills but also your ability to work under pressure. Be sure to revise long multiplication and division, percentages and conversions so that you can do them very fast.
I have just bought the OAT Maths and Physics softwares which are to the level that airlines look for. I will let you know what they are like later today...I have literally just signed for them.