Have you paid the CAA fee to the school for each of the exams you've taken thus far?
The CAA exam fee only applies to examinations taken at Examination Centres administered by the CAA. Anything charged at a Flying School is of their making. The CAA charge the examiner for the priviledge of being an examiner so they are covering their costs and time.
I have been informed by the CFI at my current flight school that the last four ground exams I passed have not been signed off and that they will charge me to have them signed off.
The school may make a charge for examinations but this should be published in their list of charges. An examiner has the responsibility to sign for an examination once complete. If they follow CAA guidance, all charges should be made prior to tests not after. CAA guidance used to be that examiners should not charge for signing documents for revalidation, most do not so simply boycott those that do.