The Hoops and who sets the jumps
I really don't think we should allow the CAA or anybody else to decide who is the right sought of chap or gal to be a Flying Instructor. Such ideas are usually subjective and will be wrong. The world is littered with very successful people who were earlier told that they would never do any good. And any way, one mans meat is another mans poison. It certainly shouldn't be for a regulatory body to restrict entrants in order to boost the income, careers or the viabilty of schools. Economics do that already. The prices and wages are set by what the man in the street has left in his pocket.
The suitablity of an Instructor to instruct should be an essentiall element of the training and will be assessed for the required standard by test. The pre-course test is simply to ensure that the potential instructor has the required minimum knowledge and basic flying ability to be further trained and as such it cannot and should not be a filtering mechanism.
The point about the CPL exams is not that they are difficult but that they are not relevant to instructing although certain elements of the exams are. A potential pilot instructor may have passed the CPL/ATPL exams but still not have seen a POH or how to research the ANO or AIP. They are usually given extracts.
A dedicated FI exam and test preceded by proper ground training to that end will be better to ensure that all instructors have the required relevant knowledge needed to pass onto their students. The ground course should not be based on feedback questions and exam technique but a considered, thoughtful progress to the standard of lnowledge and how to pass it on. A lot more emphasis must also be made on how to brief (without powerpoint) clearly and succinctly while maintaining the students attention - that is to teach. Power Point is an aid but not a method! I'm appalled sometimes at the standard of briefing that I've witnessed at various seminars and God knows what they say to the student in the aeroplane..