I did my interview for NATS in 1967 but I was told:
Read the national news papers on the way to your interview and digest anything aviation however insignificant.
Try to do something connected with aviation; I was a member of the Air Training Corps (now called Air Cadets) and was training to be a gliding instructor.
There were no 'selection tests' as such.
I was selected; did 2 1/2 years as an Air Traffic Control assistant, got promoted to Air Traffic Control Officer.
If you elect to join via the RAF, I think career progression is a bit quicker however there were selection tests with the RAF.