If you're a British citizen you may enter Germany.
If you fly directly from Thailand there are no restrictions. Incidentally there are currently also no restrictions for Thai citizens flying directly from Thailand.
If you transit Qatar then enter Germany, you must travel directly to the place where you will stay and self-isolate until 14 days have passed since you departed Qatar. Shortly you will be required to take a COVID-19 test and potentially the period of self-isolation may be reduced if negative.