Is it correct to assume RAF controllers of such devices are always aircrew?
I'm not quite as sure as 5F6B. I would say generally yes. However, a few years ago, a trial was undertaken to put a number of non-aircrew types through UAV pilot training, and initial applications were I believe accepted from pretty well any Branch. Those selected would undergo the basics of flying training and some instrument training before converting to Reaper. I'm not sure what happened, whether the trial ran as planned and spat some non-aircrew types into the UAV system or whether it was binned.
However, whilst RAF controllers are generally if not always aircrew, I know the USAF now have a specific UAV pilot career stream that anyone can apply to and become a type-rated UAV pilot. Given how the RAF seems to try and copy everything the USAF does in some way shape or form, maybe it won't be too long before our UAV drivers are spotty 12 year olds sitting in a grubby bedroom somewhere.