The CAA's guidance has always been to collect fees in advance however; I note that the last 2 editions of the FEH only refer to tests where a fee is due to the CAA. I think most Examiners collect it post flight.
Guidance on test fees might be regarded as a cartel, so there is none. You can start with the published CAA fee and adjust accordingly. About £160-180 for a PPL tests seems to be the norm.