I controlled at the northen unit and flew out of the neighbouring place as a student PPL. Here's my take on things for what its worth.....
It might have changed but when i was at that certain northern unit, at the time we started the service for the highland international airport, it was written into the flying order book that on certain depature routes the pointy pilots would accept 'not above' if there was conflicting in/out bound traffic. Even if they were FIS.
I don't remember ever imposing a not above on anyone 'cause if they are FIS they can do what they want.
It took a couple of close calls for people to accept that there was possibly a reason for the request and it became almost routine to ask VFR light ac to go no further west than a certain point and then not restict them at all (in height) if there was any remote possiblity that they would 'conflict' with the IFR.
But as this was several years ago - practice could have changed.
RATT fam flights are very very hard to come by - most controllers (especially Mil) don't get any [well not civi anyway].
TFB takes cover and retreats behind his shiney new MATS Pt1 !
edit - After thought, if there was something in the way the IFR ac usually got held at the hold as we were unwilling to let it get airborne. Sometimes they were launched anyhow and freecalled us in the climb normally in confliction with the one we were trying to prevent confliction with! - Go figure!!