Rachel,
If you want to go that route why not try for a full ATCO course first.
Try the NATS website...
Do not worry about maths too much. Mental arithmetic is more important. Try adding up bills and multiplying cost per litre, when filling your car with petrol. It's just a case of practice.
I can't comment greatly, I am in the application process for NATS myself. Feel free to PM me, I'll help as much as I can.
L8rs
J