India42:
The avionics upgrade was a result of a lot of student failures late in the course. With the original fit, instrument approaches were ground controlled (QGH to GCA or non-precision radar approach) and the bulk of the IF syllabus was towards the end of the course. Typically, a weakish student could get to about 120 hours on the 160-hour course and then not get through the IF phase. Expensive, and disappointing all round.
The ILS/VOR/DME fit was intended to increase the workload on the student pilot, and the IF was moved (mostly) to an earlier stage of the course. This all happened several years after my time as a JP QFI, but I understand it did work much as intended. One problem with the JP as a trainer was it was rather undemanding - an average result could be achieved with not too much effort or skill, but of course application and determination did produce better results.