Mike Cross, my understanding is that (apart from the few who have the old rights conferred on them by the CAA) a PPL holder cannot accept remuneration for instructing, or anything else. Now when somebody refers to a "paid PPL instructor" it is not entirely clear whether they are referring to a instructor who teaches to PPL standard or an instructor whose qualification is PPL! I took it to be the latter.
I am not disagreeing with you in what should be the case and what is fair but, under the current rules, I cannot see the point of going straight from PPL to FI without doing the CPL flight course in between. The difference in cost would be the cost of an instructor for the CPL course (given that you would have to pay for self fly hire for hour building to whatever hours are required for an instructor rating). The exams still have to be written.
However, if you are to be paid as a driving instructor you should have a PDI or ADI licence!!!
Cheers
Whirls