I would expect an acknowledgement of the instruction to squawk Ident, as you say, it is a transponder setting.
The glossy book, if it's the same one I received, contains errors, omissions and contradictions!
Normally at Aberdeen we identify prenotified departures within 1nm, as 10W mentioned. However, a few months ago we had to use a remote radar head for a while, and we then had to use the Ident on all departures. The use of the Alanshill radar also led to an increased radar separation requirement from 3 nm to 5 nm, with a commensurate increase in the need for holding and/or extended routing on arrivals. Thankfully, the usual Perwinnes radar is back and "normal" service has been resumed.