Germany has now placed the UK on its list of "Virusvariantengebiet", which is the highest risk category in the international assessment system.
From 0 o'clock on Monday morning for two weeks, until at least 3.1.22, only German citizens and residents (and their families) are allowed to enter Germany from the UK. Transit passengers are allowed.
Those able to enter, regardless of vaccination status, must have a negative test before travelling, AND must stay in home quarantine for 14 days, with no ability to take a test earlier to end the quarantine.
This is a FAQ about the rules--only in German, sorry!