<<I don't know where the selection comes from to be honest, but then where does the selection for aerodromes come from?>>
It comes from NATS knowing where staff are needed and allocating them accordingly. I don't know how it works now but yonks ago postings often depended on how well one fared on the course. The bright boys went to busy places and those less able went to quieter ones. It didn't always work out though because nobody can tell if a student will validate anywhere until he/she starts live training.