Well... I know a friend, he did 2 times the ATPL exams (UK and Dutch). He did the CPL course finished up the CPL. Some years later he did the IR course but the theory was not valid anymore. He got an extension from the dutch caa and finaly the uk caa accepted the dutch theory. Maybe there is a way that they accept the german theory.
Call the uk caa and ask them. They will help you out. Could be that they accept german theoretical exams.
You are right, there is no point of teaching in german and doing the exams in english. But the problem is, the translation of the german question is so bad! All students at my school they do the exams in engish except HPL.