The only credit one gets on the JAA exam side of things is that the holder of an ICAO IR may not have to attend the JAA IR mandatory ground school (classroom). It is at the "discretion" of the head of the school. This means you can self study, and just turn up at Gatwick and sit the exams. There is a valid strategy which does work for clever pilots who already know the practical stuff: swat a bit, sit all the exams in one go, and then revise seriously for those which you failed and re-take them. This won't work for "ab initio IR" pilots because most of the study material is totally alien stuff to a pure PPL.
Pace is right but we live with European elitism, protectionism, and gold plating.