The fact that you are a German national does make things slightly easier. And although things have been tightened up a fair bit in the last few years, the UK is still a place where it's relatively easy to pretend to be resident when you're not because there is no need to register one's place of residence with the authorities here. We had a "phantom resident" at our home for years - a German friend who was using our address for his Sky subscription.

For things like opening a bank account they will now typically ask for picture ID plus a utility bill (or sometimes two). I suspect it will be similar for obtaining a driving licence.

The utility bill is obviously going to be the tricky bit, unless you can persuade a friend to add your name to their gas or electricity account.

Something that might work, assuming that you have a friend's address that you can use, is to go and buy a television in the UK. The shop will ask for your address (they are required to by law), and two or three weeks later you will receive a bill for a TV licence fee from the TV Licensing people. With a bit of luck this will suffice as proof of residence.

Obviously all this will take a few weeks... and then there is also the added complication that you will have to send in your passport (or take it to Swansea) to be issued a driving licence.

The above would probably work... but it would be a lot of hassle, and would take a long time, and would require at least one trip to the UK but probably more. It may work out easier and cheaper to re-take your test.