I've not come across that before; how is the £100 justified? The school makes their money on SFH aircraft rental, the examiner gets paid, and then £100 ... admin?
In the schools I have worked for, they often viewed an LPC as a customer relations exercise, not a profit making one. External examiners were paid a flat rate for the day, and only booked when there were atleast sufficient students to cover costs (never a problem). Else it was the CFI who was salaried.