As you will no doubt find by reading many of the strings in this forum, there are probably many ATCOs currently employed by NATS who might find it difficult to think of an answer to your first question.
The company, like any potential employer, is looking for an answer that shows that you aren't applying because "There was nothing good on the telly," or "A bloke at the pub thought it would be fun." Rather they are looking to invest many thousands of pounds training someone who has researched what the job entails, and has arrived at the desicion to apply based on sound thought processes. If you can't come up with a structured answer, I'd ask myself, were I you, if I had the commitment to go through one of the toughest courses outside the military.
The answer to the second question is that your referees should be people in positions of responsibility, who know you; Teachers, Mentors, Doctors, Magistrates, Business contacts, professionals etc. An ATCO would be just fine.
Pimps, madams and fellow gang-members are not usually considered reliable.