When throwing a paddy, could everybody please make it clear whether they are arguing against instructors being unqualified (fair enough) or just unpaid. There is a difference, and I assume that since an FI rating costs roughly the same as a CPL course, then the latter of those points is hardly valid. If the CAA say you're competent to instruct, then either they're being slack, or you're competent.
That brings us purely to the question of paid or unpaid (A personal choice, I would have thought, as long as you are qualified). Whatever points you may raise about this, selfish or otherwise (or valid or otherwise), please don't cloud the issue by talking about qualifications, because it's not relevant to the same extent.