Read this:
http://www.expatica.com/uk/visas-and...y_1029503.html
Read all of the text, but for a start, as a German national, your partner has the right to travel to the UK, look for a job, work and live in the UK without a visa. Partnerships/marital status is not an issue to do any of these things. But every member state in the EU has the right to impose additional regulations that do not violate the rights of members of EU to reside and work/or not in the UK and these are spelled out in the URL. Every state is slightly different than that regard, but you can worry about that later, after your partner arrives. She will need to travel with her German passport and and any other official identification documents/cards, etc. issued by the German government and state governments (like a drivers license).
So, there is no immediate hassle. The big question is what will happen once the UK LEAVES the EU, and this is somewhat up in the air, although the UK has promulgated some regulations already about this to protect the status of of EU "residents" in the UK, and they are available on the net.
All this means, basically, is that your partner is NOT considered an immigrant...as long as he/she is a citizen of the UK, no matter how that citizenship status was granted, by birth or naturalization.