like having to sit the massive ATPL ground school, just to be able to be a paid PPL instructor
A paid PPL instructor is an oxymoron like Fun Run or Military Intellligence!
A instructor holding a PPL only cannot be paid (unless they are one of the few left who still have the grandfather rights from the olden days of CAA).
You do not have to sit the 14 ATPL exams; you can sit the 9 CPL exams.
Whilst the old CAA system (as noted by
BEags and
Mike) was much simpler, there is no advantage to going that route at the moment.
Using helicopter requirements ('cos that's all I know but fixed wings is similar), one must build one's hours up to 250 in order to be able to take an FI course. In the meantime, one must pass 9 (or 14) exams. However, at 155 hours, one is eligible to take a commercial flight course thereby gaining a CPL. Since the hours for the CPL count towards the 250 for the instructors course, one might as well do the CPL (the only difference in cost would be the dual hours with an instructor and licence issue) and then you could be paid.
However, I do think that if one can only get a Class 2 medical, then one can instruct without renumeration on a PPL.
Cheers
Whirls