Can you tell me who is suitable to be an ATCO?
Well, the aptitude tests/assesments done by the two independant colleges and CATC at Hurn are by no means infallible, but they do provide a very good indicator as to who will get through the courses and who will not, even CATC's output of successful candidates is markedly better now.
CATC and the two independants will tell candidates immediately if they don't think they'll make it through the course. Their assesments are done by instructors who have to maintain CAA standards to keep their instructors tickets.
It's a well known and proven fact that it's better for the candidates to go into the interview and test days having done the recommended reading, visits and with an open mind, which they can do for free - not having paid for the "advice" of someone who appears not to be a qualified instructor, nor is an atco.