Under these circumstances, is transiting in Schiphol permitted?
Are there additional demands, ie a recent negative test?
Strictly speaking you also need to fill in the 'traveller public health declaration' which may be downloaded here. As I explained in a comment, I have not been asked for those forms in AMS (both on arrival or departure). Nevertheless, it may be something that the airline could check when you're checking in (as it's also listed in Timatic).
I also recommend looking at the German entry requirements as well. Also note that the Netherlands is currently marked red on the ECDC map.
The Netherlands has an exception to the mandatory negative test result requirement when changing planes:
Transit/transfer in the Netherlands
If you start your journey in a high-risk area or country and change planes in the Netherlands
- You do not need to show a negative NAAT (PCR) test result or rapid test result on arrival in the Netherlands.
- A transfer is when you change planes and continue your journey within a few hours, and in any case within 1 day without leaving the airport where you transfer.
It's not entirely clear from the English translation that this applies to travel originating from the UK. The Dutch version of the same page describes the same rules using the qualifier an area with an orange travel advisory. At this moment, the UK does have an orange travel advisory.