In the phraseology section of MATS, it says:
"Descend on the ILS, QNH (pressure) millibars, elevation (number) feet."
Most units dont give the QNH and elevation and just "descend on the ILS".
(in the college of knowledge, we had to say it all the time when we cleared someone for the ILS - it got annoying, like you said, the pilot should know it, it is there on the chart)
Perhaps Prestwick has been told for some reason to start doing it?
Just a guess...Correct me if I am wrong