ADe - I would suggest that you propose the following plan to the Flugschule Hamburg:
1. All theoretical examinations will be conducted in English under the invigilation of a CAA-appointed UK Examiner, either in England or in Germany. The examination results will be recorded on the appropriate CAA PPL application form signed by the Examiner for you to attach to your German application.
2. In accordance with JAR-FCL 1, you will obtain your medical certificate in Germany.
3. In accordance with JAR-FCL 1, you will complete all flying training and the JAR-FCL PPL(A) Skill Test in Germany.
Make sure that you get written approval both from the UK CAA (probably from the policy department at the Safety Regualtion Group) AND from the German civil aviation authorities.
Regarding the cost of a UK examiner coming to Hamburg, that's one of those variables which it's impossible to quantify. The CAA appoints 'examination paper custodians' (such as me!) who are totally responsible for the security of their papers, so they would be highly unlikely to send any copies to a 3rd party! I would suggest that you first obtain the approval for the plan I've outlined above, then decide where to sit the exams. A home study course for the PPL(A) theory should present little problem, many PPL students now do all their theoretical study from reference books, backed up with tutorials from Flight Instructors.
Fees for PPL(A) exams are entirely down to individual Examiners. For example, I make no charge whatsoever for first attempts; however, for second attempts I will charge €15 per paper to ensure that the applicant does some more studying first! Anyone who fails a paper twice will be advised to contact an a specialist ground training organisation before sitting the exam for the third attempt at that organisation. If the third attempt is unsuccesful, the exams have to be sat at the CAA Gatwick at about €55 for each exam, but only after a certificate has been signed stating that the applicant has undertaken a furthe course of study. But don't get too worried, most people pass at the first attempt...
Another point which you need to address is the training and testing required for whatever radio licence the German authorities require at PPL level. In the UK, only approved RT Examiners may conduct the practical test; however, the theoretical communications exam can be set by any approved ground examiner. If the Germans have a similar system, perhaps you could do the theory exam in the UK but the practical in Germany?
Keep us posted with the progress of your plan - and the very best of luck!