I don't agree with having to pass the CPL written exams before being allowed to start an instructors course. While some of the information is relevent much of it isn't. If particular knowledge is required to be a good instructor then I think this should be part of the instructors course.
The cost of the CPL writtens is no longer trivial. I took the CAA exams five years ago, and I am now taking the JAR version of the CPL writtens so I am in a position to compare the cost. The cost has risen enormously. The cheapest route (distance learning) now costs well over £2000 assuming first-time passes in every subject, and the cost of an instructor rating course has risen to nearly £5000. So the cost of the CPL writtens is now of the order of 30% of the total cost of getting an instructor rating.