Firstly, 170A is a form to be filled out by a CAA qualified person. NOTHING more. It is to confirm that you are suitable to sit the relevant skills test. Both in terms of competance AND paperwork/experience.
Now, if the 170A signatory has never seen you fly, then obviously he will want to do so. Logically, in the form of a practice test. However, if they were your instructor during the course, this may not be required.
Bottom line. 170A is just a form which needs to be signed before test. They need to see you reach test standard. There is NO reason why this can't be done as part of your course (thus, NO extra charge). If he is charging £100 to sign the form, that is nothing short of extaution. If you choose to pay and he doesn't give you a reciept, then I'm sure that Her Maj's Inland Revenue AND the CAA would be interested about these 'non - standard' operating/accounting procedures.
If they advertise that the 170A signature has a fee and you choose to pay, then that is fair enough. I wouldn't though. There is no reason why the student should pay for the 170A's expenses. It is an overhead of running a FTO. When did you last get charged for your intructors class 1 medical? Besides, £100 is outrageously OTT.
EK