1. It's a NST, not NFT.
2. What you charge for Skill Tests is your own business; however, I charge £150 for a JAR-FCL PPL Skill Test or £75 for either a NPPL NST or GST. So the cost is the same - unless you're charging large travelling expenses.
3. The CAA 'recommends' taking the exams at intervals throughout the JAR-FCL PPL course so that an applicant's theoretical knowledege backs up the stage of flying they've reached. The only manadatory requirement is that all exams must have been passed before the applicant may take the JAR-FCL PPL Skill Test. It's the same for the NPPL and no, the theoretical exams do NOT have to be completed before the applicant takes the NST. Individual RFs and FTOs may - as I do - impose local rules to ensure that the applicant's theoretical knowledge keeps up with the flying training stage reached.