Long distance relationship: UK resident staying in Germany for 6 months? [closed]

As Peter pointed out in his answer, as an EU citizen you enjoy freedom of movement in the EU. We can only guess when a Brexit will take effect, but it is quite unlikely that changes will take effect in late 2017 or early 2018.

• The freedom of movement was intended to cover the movement of workers, but it also covers other economically self-sufficient EU citizens.
• If you become a resident of Germany, you must register with the municipal authorities for tax purposes, elections, etc. (You can vote in some elections, not all.) This may be required within 14 days if you are "moving in" with your boyfriend in an apartment.
• The people at Expatriates Stack Exchange may be able to help with health insurance questions.
• If you have significant income or assets, you should talk to a lawyer or accountant about the details of moving your primary residency.

As of right now the UK is in the EU so you can visit for as long as you like with no visas.

This is likely to change in the forseeable future but what exactly the arrangements will be is not known at this point.