I wouldn't really get too worked up about those tests. I think they are designed to test the natural aptitude, not the studying and preparation.
I remember going in very relaxed, without expectations. Half way through I was tempted to leave the room since I thought I was doing so bad I was just wasting my time. Everybody else always seems to be doing better and quicker, but then again they could be picking wrong answers.
So I stayed and finished the tests ad here I am off to the college.
I wouldn't worry too much about the dress either. I did my tests in casual and my interview in ASDA George gear. Worked fine. Your skills and personality say a lot more about you than what you are wearing !
I've visited 3 NATS units and haven't seen a big deal of dress code. In fact one of the ATCOs I sat next to was wearing shorts
(and yes it was winter. In the North
)
good luck
Ivor