I learned my German through the Goethe Institut and can wholeheartedly recommend them. More to the point, they have well-researched and well-resourced programmes of study, and so you can carry on seamlessly from one location to another as they all have the same definitions and standards of teaching level, so that if you are at a B1 level in Hannover and you move to Berlin, you can continue on the same B1 level there, and continue to progress starting from where you left off.

And there are Goethe-Instituts outside Germany. I studied at the one linked to Nancy University in France, and I happen to know that there is a branch in Manchester, UK. They also offer exams which are considered to be correct in their level of assessments and are considered as valid statements of ability level in countries even outside Germany.