I would have thought that your local Adult Education Centre would be a good place to contact about GCSE courses. If they are not much help, try asking local Further Education colleges in your area, as although geared towards 16-19 year olds, many also do courses for Adult Learners.
Example, Wolverhampton College offer GCSE Mathematics
Another alternative would be Distance Learning.
Oxford Open Learning provide both GCSE Maths and GCSE English.
I am surprised that the OU can not advise regarding alternative entry requirements for thier PGCE, surely they can take into account courses that you have done in Germany. Is there a specific PGCE course you are intending to do (please let me know the OU course reference if available).
OU PGCE - Course Code C21 : Entry Requirements
Have a degree from a UK university or an equivalent qualification that is clearly and substantially relevant to one of the six secondary PGCE subjects. This means that at least half and preferably two-thirds of your degree (at least 180 credit points) should comprise studies relevant to the subject you intend to teach.
Have (or expect to have by the time you begin the course) a GCSE grade A?C, or the equivalent, in English language and in mathematics.
I presume that you have a suitable Degree and that it is the GCSE grade A-C requirement that is the sticking point. OU do say or the equivalent so if you have done courses in the past which you feel would be equivalent, do contact the OU and try to persuade them to accept your past qualifications.
Hope that is of some help.