Unfortunately, pressman's idea won't work. To change the State of Licence Issue, you must have a medical certificate issued by or rendered valid by the receiving state. If the CAA medical branch have stated that they will not accept your German medical then that will still be the case if and when you have a German (or any other State's) issued JAA CPL. Regrettably, it looks as though the UK can never be your State of Licence Issue.
Interesting that the same UK medical branch recently issued a Class 2 JAA medical to a person who had previously been refused by the French DGAC on the grounds that (s)he is profoundly deaf and, as a consequence, is unable to speak.