Drizzle,
Whilst you are possibly correct with respect of unpaid instructors, that wasn't (I think) what GKEST was referring to.
Like many reading these forums I started out instructing on a ppl, for which you were allowed to get paid prior to 1988 and the introduction of the BCPL. Looking back to those times and looking at what is happening now I see no real difference in the standard of instruction at the club which I was (and still am) associated with. However, I accept that others may have different experiences, of course.
Is there any evidence that the standard of instruction at PPL level in the UK is improved by the instructor having a detailed theoretical knowledge of world climatology or the workings of an inertial navigation system?
Personally I believe the answer is no, and that an instructor holding a PPL and IMC rating is perfectly capable of instructing to PPL level if they are competent in handling light aircraft and have been through a suitable instructors course - indeed for some exercises, aerobatics and tailwheel training spring to mind - the commercial written exams or flight test have absolutely no relevance.
That said I admit that for the IMC rating I probably wasn't really ready to start teaching it when I did, and for more advanced ratings an IR and some experience of IMC operations will make the instructors life a lot easier, and improve the quality of instruction received by the student.