You need to decide where you will be resident and pay your taxes accordingly, you can only pick one. Talk to an accountant who knows Germany as their taxes are quite different to the UK, which is best will depend on your exact situation.
If your friend knows you are abroad and pays you rent she is actually required to deduct basic rate income tax from it, unless you're formally registered as a non resident landlord. Google NRL1i for starters.
You need to decide where you will be resident and pay your taxes accordingly, you can only pick one. Talk to an accountant who knows Germany as their taxes are quite different to the UK, which is best will depend on your exact situation.
If your friend knows you are abroad and pays you rent she is actually required to deduct basic rate income tax from it, unless you're formally registered as a non resident landlord. Google NRL1i for starters.
Where in Germany? Free forum holiday to the alps?