Just out of interest, it used to go something like this..............
When a vacancy arose for an ATSA at BRS, it was advertised around the airport staff and business partner staff (BA, Easy, Servisair etc).
Then when we decided to recruit an ATCO internally, the position was only open for our ATSA's to apply for. The last one who got the job had to sit through the NATS selection tests anyway (cos that's who the management paid to assess which ATSA was suitable).
Competition was fierce and had we stayed non-NATS, there was less chance of someone becoming an ATCO down that route than applying directly to NATS.