The CAA do not make any charge for PPL exams (unless they have introduced one very recently). They recommend a fee of £45 per exam. This fee does not go to the CAA, but is retained by the school that is providing the exams.
Schools are permitted to charge any lower fee if they wish. In some cases schools do not charge any exam fee for candidates who are also carrying out their flying training with them. The same schools may charge the full £45 for each exam for candidates who intend to do their flying elsewhere.