All the PPL exams in the UK are the copyright of the CAA and are NOT available elsewhere. The questions in the PPL Confuser etc are similar, but not the same as the real questions. It is a good revision tool, but you also need to study the books first.
The reason the CAA won't accept passes in PPL exams conducted other than at CAA-approved RFs/FTOs is that they have no control of the custody and management of 'foreign' JAR-FCL PPL exams. They will, of course, accept passes from JAR-FCL PPL exams sat in the USA at schools approved by the CAA for JAR-FCL PPL training.
As for costs, our Club does not charge our own students for the first attempt. But anyone else who just wants to sit the exams before going elsewhere to do their flying training will be charged the same rate that the CAA charges!